[docs] coverage: Clarify which flags enable gcov-style profiling (NFC)
[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: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@294626 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang