Update documentation and release notes for llvm-profgen COFF support (#84864)

This change:
- Updates the existing Clang User's Manual section on SPGO so that it
describes how to use llvm-profgen to perform SPGO on Windows. This is
new functionality implemented in #83972.
- Fixes a minor typo in the existing llvm-profgen invocation example.
- Adds an LLVM release note on this new functionality in llvm-profgen.