Merge branch 'ls/travis-doc-asciidoctor' into maint

Travis CI gained a task to format the documentation with both AsciiDoc and AsciiDoctor. * ls/travis-doc-asciidoctor: travis-ci: check AsciiDoc/AsciiDoctor stderr output travis-ci: unset compiler for jobs that do not need one travis-ci: parallelize documentation build travis-ci: build documentation with AsciiDoc and Asciidoctor