Merge branch 'mb/config-document-include' into maint

Add missing documentation for "include" and "includeIf" features in "git config" file format, which incidentally teaches the command line completion to include them in its offerings. source: <pull.1285.v2.git.1658002423864.gitgitgadget@gmail.com> * mb/config-document-include: config.txt: document include, includeIf