Clarify the purpose of FEATURE_precompile_header

The top level configure.cmake in qtbase has:

qt_feature("precompile_header"
    LABEL "Using precompiled headers"
    CONDITION BUILD_WITH_PCH AND TEST_precompile_header
    AUTODETECT NOT WASM
)

but absolutely nothing uses FEATURE_precompile_header, only BUILD_WITH_PCH.

This is suspicious. For instance, if the associated test fails, is the FEATURE going to be "off" but Qt would still be using PCH...?