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
linux amd64
autoreconf

00:00:18

 
  Precheck
passed #1443243
linux amd64
clang-format

00:00:18

canceled #1443244
linux amd64
coccinelle

00:02:25

passed #1443245
linux amd64
danger

00:00:14

passed #1443242
linux amd64
misc

00:00:20

passed #1443246
linux amd64
tarball-create

00:01:29

 
  Build
canceled #1443259
linux amd64
clang:asan

00:00:32

canceled #1443263
linux amd64
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
linux amd64
clang:tsan

00:00:45

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

00:00:45

canceled #1443258
linux amd64
gcc:asan

00:00:37

canceled #1443251
linux amd64
gcc:buster:amd64

00:00:44

canceled #1443248
linux amd64
gcc:centos7:amd64

00:00:48

canceled #1443249
linux amd64
gcc:centos8:amd64

00:00:47

canceled #1443257
linux amd64
gcc:focal:amd64

00:00:42

canceled #1443262
linux amd64
gcc:mutexatomics

00:00:51

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

00:00:45

canceled #1443252
linux amd64
gcc:sid:amd64

00:00:47

canceled #1443254
linux i386
gcc:sid:i386

00:00:46

canceled #1443264
linux amd64
gcc:softhsm2.4

00:00:52

canceled #1443265
linux amd64
gcc:softhsm2.6

00:00:52

canceled #1443250
linux amd64
gcc:stretch:amd64

00:00:52

canceled #1443260
linux amd64
gcc:tsan

00:00:46

canceled #1443255
linux amd64
gcc:tumbleweed:amd64

00:00:54

canceled #1443256
linux amd64
gcc:xenial:amd64

00:00:55

passed #1443269
amd64 windows
msvc:windows:amd64

00:00:46

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

00:01:06

 
  Postcheck
canceled #1443315
linux amd64
cppcheck

00:00:33

passed #1443312
linux amd64
flake8

00:00:19

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

00:00:24

canceled #1443314
linux amd64
scan-build

00:00:34