Help: Do not recommend WCDH in cmake-compile-features(7)

Since commit da7ad799 (WriteCompilerDetectionHeader: Add policy to
remove module, 2020-12-04, v3.20.0-rc1~350^2), the WCDH module is
deprecated.  Update the `cmake-compile-features(7)` manual section that
previously recommended WCDH to make such detection the project's
responsibility instead.  Move the old content of the section over to the
WCDH module to preserve it.