Help: Clarify behavior of path-based equality conditions

The docs for if(PATH_EQUAL) did not mention the component-wise nature of the checks. It also claimed no normalization was performed, but multiple separators are effectively collapsed (part of normalization). Improve the wording to clarify both of these points.

Also update the corresponding description of cmake_path(COMPARE), which had the same inaccuracy.

Fixes: #23758 (closed)
Backport: release