doc: fix mansrc generation

[hzhou]

Pull Request Description

Just using the name without prefix, e.g. MPISend -> send.txt, resulting in duplicated filenames between MPI/MPIT/MPIX_ functions, and they overwrite each other. This resulted in certain man pages missing. For example, MPI_Finalize and MPI_Init_thread man pages were missing because they were overwritten by the MPIT correspondents.

The fix is to use the whole name for mansrc files, e.g. MPI_Finalize.txt and MPI_T_Finalize.txt.

Fixes #7089 [skip warnings]

Author Checklist

• [x] Provide Description Particularly focus on why, not what. Reference background, issues, test failures, xfail entries, etc.
• [x] Commits Follow Good Practice Commits are self-contained and do not do two things at once. Commit message is of the form: module: short description Commit message explains what's in the commit.
• [ ] Passes All Tests Whitespace checker. Warnings test. Additional tests via comments.
• [x] Contribution Agreement For non-Argonne authors, check contribution agreement. If necessary, request an explicit comment from your companies PR approval manager.

[hzhou]

I locally verified the man pages for MPI_Finalize and MPI_Init_thread are generated after this PR.

[hzhou]

Somehow the CI triggers are stuck. I'm going to merge anyway since the fix is trivial and won't be covered by the CI anyway.