Doc update for feature request: pre-compiled headers with clang

[anadon]

With regard to https://boostorg.github.io/build/manual/master/index.html#bbv2.reference.precompiled_headers

clang has supported its own kind of pre-compiled headers. Should this documentation be updated, or has this feature not been implemented?

[stale[bot]]

Thank you for your contributions. Main development of B2 has moved to https://github.com/bfgroup/b2 This issue has been automatically marked as "transition" to indicate the potential for needing transition to the new B2 development project.