[Docs] [llvm-mca] Point out a caveat for using llvm-mca markers in source code.

Summary: See: https://bugs.llvm.org/show_bug.cgi?id=42173

Reviewers: andreadb, mattd, RKSimon, spatel

Reviewed By: andreadb

Subscribers: tschuett, gbedwell, llvm-commits, andreadb

Tags: #llvm

Patch by Max Marrone (maxpm)! Thanks!

Differential Revision: https://reviews.llvm.org/D63040

llvm-svn: 362979