docs: correct incorrect types for pch files
docs: correct incorrect types for pch files

In the original RefMan 2.0 implementation, the types for this were
filled in as `str | file`, but the code only ever accepted the former.
Fix the documentation so that it aligns with reality.

Fixes #10338

pexip/meson,pexip/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,pexip/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,pexip/meson,pexip/meson,mesonbuild/meson,pexip/meson