[clang] [Clang][PGO][UserManual] Specify usage of -b flag (PR #139763)

[Tim Creech via cfe-commits]

https://github.com/tcreech-intel approved this pull request.

Thanks! Looks good to me.

Step 2 mentions `-b`, but makes it sound optional because it was optional with AutoFDO. Llvm-profgen doesn't have a mode to not require LBRs and this is good to point out.

https://github.com/llvm/llvm-project/pull/139763