[Support] Update comments about stdout, raw_fd_ostream, and outs()

The full story is in the comments:

  // Do not attempt to close stdout or stderr. We used to try to maintain the
  // property that tools that support writing file to stdout should not also
  // write informational output to stdout, but in practice we were never able to
  // maintain this invariant. Many features have been added to LLVM and clang
  // (-fdump-record-layouts, optimization remarks, etc) that print to stdout, so
  // users must simply be aware that mixed output and remarks is a possibility.

NFC, I am just updating comments to reflect reality.

llvm-svn: 310016