docs: fix refman 2.0 regression in correctly describing find_library(has_headers)
docs: fix refman 2.0 regression in correctly describing find_library(has_headers)

All kwargs inherited from has_header need to be prefixed `header_` so we
cannot just do straight inheritance. And the part of the description
that highlighted the way kwargs are derived and evolved, went entirely
missing.

Fixes #9551

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