[v9_11] Lint manual pages
As we generate manual pages from DocBook sources, we don't have absolute control on manual page output and therefore 'mandoc -Tlint' may always report warnings we can't eliminate. In light of this linting level was set for mandoc to '-Werror' to ignore warnings and style issues, only errors are reported.
(cherry picked from commit 22fdcb30)
Closes #2312 (closed)