Merge topic 'doc-cmake-build-signature' into release-3.21

Help/manual/cmake.1.rst

Source/cmakemain.cxx

...mmandLine/BuildDir--build--parallel-bad-number-stderr.txt

...ke/CommandLine/BuildDir--build--parallel-large-stderr.txt

.../BuildDir--build--parallel-no-space-bad-number-stderr.txt

...ake/CommandLine/BuildDir--build--parallel-zero-stderr.txt

...ke/CommandLine/BuildDir--build-jobs-bad-number-stderr.txt

...unCMake/CommandLine/BuildDir--build-jobs-large-stderr.txt

...dLine/BuildDir--build-jobs-no-space-bad-number-stderr.txt

...RunCMake/CommandLine/BuildDir--build-jobs-zero-stderr.txt

...ldDir--build-multiple-targets-with-clean-first-stderr.txt

...dDir--build-multiple-targets-with-clean-second-stderr.txt

Tests/RunCMake/CommandLine/build-no-dir-stderr.txt

...unCMake/CommandLine/build-unknown-command-long-stderr.txt

...ommandLine/build-unknown-command-partial-match-stderr.txt

...nCMake/CommandLine/build-unknown-command-short-stderr.txt