doc: Document the workflows of the .tarball-version and .version files.
* doc/package-version.texi: New file.
* doc/gnulib.texi (Build Infrastructure Modules): Include it.
* build-aux/git-version-gen: Fix comments: Fix description of
.tarball-version. Recommend to invoke git-version-gen at configure time,
not at autoconf time. Remove description of .version. Rename Makefile
target 'tarball-version' to 'dist-tarball-version'.
* top/GNUmakefile: Improve comments.