Do not use parallel make jobs for building docs

Concurrently running sphinx-build instances started in the same working
directory are not separated from each other in any way.  This can cause
intermittent problems which we failed to notice before because they only
trigger Sphinx warnings, not errors, e.g.:

    WARNING: toctree contains ref to nonexisting file 'reference'

The message above is not triggered because doc/arm/reference.rst is
actually missing from disk at any point, but rather because a temporary
file created by one sphinx-build instance gets truncated by another one
working in parallel (the confusing message quoted above is logged
because of an overly broad "except" statement in Sphinx code).

Drop the -j command line switch from the make invocation used for
building documentation to prevent intermittent problems caused by
multiple sphinx-build instances running simultaneously in the same
working directory.

Also drop the -k command line switch from the same make invocation while
we are at it as that switch does not bring any practical value in this
case.
76 jobs for !4626 with 2448-do-not-use-parallel-make-jobs-for-building-docs in 2 minutes and 26 seconds (queued for 8 seconds)
detached
Status Job ID Name Coverage
  Autoconf
passed #1443241
amd64 linux
autoreconf

00:00:18

 
  Precheck
passed #1443243
amd64 linux
clang-format

00:00:18

canceled #1443244
amd64 linux
coccinelle

00:02:25

passed #1443245
amd64 linux
danger

00:00:14

passed #1443242
amd64 linux
misc

00:00:20

passed #1443246
amd64 linux
tarball-create

00:01:29

 
  Build
canceled #1443259
amd64 linux
clang:asan

00:00:32

canceled #1443263
amd64 linux
clang:buster:amd64

00:00:42

canceled #1443266
amd64 libvirt
clang:freebsd11:amd64

00:00:41

canceled #1443267
amd64 libvirt
clang:freebsd12:amd64

00:00:31

canceled #1443268
amd64 libvirt
clang:openbsd:amd64

00:00:29

canceled #1443261
amd64 linux
clang:tsan

00:00:45

canceled #1443247
amd64 linux
gcc:alpine3.12:amd64

00:00:45

canceled #1443258
amd64 linux
gcc:asan

00:00:37

canceled #1443251
amd64 linux
gcc:buster:amd64

00:00:44

canceled #1443248
amd64 linux
gcc:centos7:amd64

00:00:48

canceled #1443249
amd64 linux
gcc:centos8:amd64

00:00:47

canceled #1443257
amd64 linux
gcc:focal:amd64

00:00:42

canceled #1443262
amd64 linux
gcc:mutexatomics

00:00:51

canceled #1443253
amd64 linux
gcc:out-of-tree

00:00:45

canceled #1443252
amd64 linux
gcc:sid:amd64

00:00:47

canceled #1443254
i386 linux
gcc:sid:i386

00:00:46

canceled #1443264
amd64 linux
gcc:softhsm2.4

00:00:52

canceled #1443265
amd64 linux
gcc:softhsm2.6

00:00:52

canceled #1443250
amd64 linux
gcc:stretch:amd64

00:00:52

canceled #1443260
amd64 linux
gcc:tsan

00:00:46

canceled #1443255
amd64 linux
gcc:tumbleweed:amd64

00:00:54

canceled #1443256
amd64 linux
gcc:xenial:amd64

00:00:55

passed #1443269
amd64 windows
msvc:windows:amd64

00:00:46

 
  Unit
skipped #1443281
amd64 linux
unit:clang:asan
skipped #1443285
amd64 linux
unit:clang:buster:amd64
skipped #1443288
amd64 libvirt
unit:clang:freebsd11:amd64
skipped #1443289
amd64 libvirt
unit:clang:freebsd12:amd64
skipped #1443290
amd64 libvirt
unit:clang:openbsd:amd64
skipped #1443283
amd64 linux
unit:clang:tsan
skipped #1443270
amd64 linux
unit:gcc:alpine3.12:amd64
skipped #1443280
amd64 linux
unit:gcc:asan
skipped #1443274
amd64 linux
unit:gcc:buster:amd64
skipped #1443271
amd64 linux
unit:gcc:centos7:amd64
skipped #1443272
amd64 linux
unit:gcc:centos8:amd64
skipped #1443279
amd64 linux
unit:gcc:focal:amd64
skipped #1443284
amd64 linux
unit:gcc:mutexatomics
skipped #1443275
amd64 linux
unit:gcc:sid:amd64
skipped #1443276
i386 linux
unit:gcc:sid:i386
skipped #1443286
amd64 linux
unit:gcc:softhsm2.4
skipped #1443287
amd64 linux
unit:gcc:softhsm2.6
skipped #1443273
amd64 linux
unit:gcc:stretch:amd64
skipped #1443282
amd64 linux
unit:gcc:tsan
skipped #1443277
amd64 linux
unit:gcc:tumbleweed:amd64
skipped #1443278
amd64 linux
unit:gcc:xenial:amd64
 
  System
skipped #1443302
amd64 linux
system:clang:asan
skipped #1443306
amd64 linux
system:clang:buster:amd64
skipped #1443309
amd64 libvirt
system:clang:freebsd11:amd64
skipped #1443310
amd64 libvirt
system:clang:freebsd12:amd64
skipped #1443304
amd64 linux allowed to fail
system:clang:tsan
skipped #1443291
amd64 linux
system:gcc:alpine3.12:amd64
skipped #1443301
amd64 linux
system:gcc:asan
skipped #1443295
amd64 linux
system:gcc:buster:amd64
skipped #1443292
amd64 linux
system:gcc:centos7:amd64
skipped #1443293
amd64 linux
system:gcc:centos8:amd64
skipped #1443300
amd64 linux
system:gcc:focal:amd64
skipped #1443305
amd64 linux
system:gcc:mutexatomics
skipped #1443296
amd64 linux
system:gcc:sid:amd64
skipped #1443297
i386 linux
system:gcc:sid:i386
skipped #1443307
amd64 linux
system:gcc:softhsm2.4
skipped #1443308
amd64 linux
system:gcc:softhsm2.6
skipped #1443294
amd64 linux
system:gcc:stretch:amd64
skipped #1443303
amd64 linux allowed to fail
system:gcc:tsan
skipped #1443298
amd64 linux
system:gcc:tumbleweed:amd64
skipped #1443299
amd64 linux
system:gcc:xenial:amd64
 
  Docs
passed #1443311
amd64 linux
docs

00:01:06

 
  Postcheck
canceled #1443315
amd64 linux
cppcheck

00:00:33

passed #1443312
amd64 linux
flake8

00:00:19

skipped #1443316
amd64 linux
gcov
passed #1443313
amd64 linux
pylint

00:00:24

canceled #1443314
amd64 linux
scan-build

00:00:34