Update tuning_parameters.md

[rithwiktom]

Pull Request Description

This PR updates/adds description for some MPICH CVARs

Author Checklist

• [ ] Provide Description Particularly focus on why, not what. Reference background, issues, test failures, xfail entries, etc.
• [ ] 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.
• [ ] Contribution Agreement For non-Argonne authors, check contribution agreement. If necessary, request an explicit comment from your companies PR approval manager.