[docs] coverage: Clarify which flags enable gcov-style profiling (NFC)

Point out that --coverage and -ftest-coverage, which is what most people
are used to, do not enable clang's frontend based coverage pass.

Suggested by Benn Bolay!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@294626 91177308-0d34-0410-b5e6-96231b3b80d8