Merge topic 'doc-cleanups-3.21'

8d1944c675 Help: Expand details for file(COPY_FILE)
c8ea886123 Help: Re-order file() sub-commands in Filesystem section
b5a96716eb Help: Clarify which project() call PROJECT_IS_TOP_LEVEL is for
8be7694d97 Help: Fix trivial typo

Acked-by: Kitware Robot <kwrobot@kitware.com>
Merge-request: !6361