Documentation incomplete for qmake and precompiled headers

In qmake's Using Precompiled Header documentation there are a few things missing:

1. Unix section lists a compiler but OS X and Windows sections list build schemes
   • (Clang works for Linux and probably for OS X too)
2. "you only need to..." isn't correct
   • (there are two other things needed)
3. config flag precompile_header not described
   • (but it's in the example)
4. variable for PRECOMPILED_DIR not mentioned
   • (without it, qmake picks your target name, which causes a bizarre compile error because of the file/directory conflict)
   • (maybe this needs a qmake code change)