The mechanics of using precompiled headers was changed by JDK-8347909. The precompiled.hpp header is now automatically included when configured to use it. That ought to be stated explicitly.

The current style guide text says:

Some build configurations use precompiled headers to speed up the build times. The precompiled headers are included in the precompiled.hpp file. Note that precompiled.hpp is just a build time optimization, so don't rely on it to resolve include problems.

Suggested new text - something like:

Some build configurations use precompiled headers to speed up the build times. Use of precompiled headers is just a build time optimization, so don't rely on it to resolve include problems. The precompiled headers are included in the precompiled.hpp file. That header is included automatically when configured to use it; it should not be included explicitly.