Merge topic 'doc-signatures'

3600be4e31 Help: Use signature directive for 'if' command
bedd62d52c Help: Improve sentence structure around "if so"
ae7c4a5ff4 Help: Add internal cross-references to 'if' command docs
95bfacb8bb Help: Indent 'if' command docs more consistently

Acked-by: Kitware Robot <kwrobot@kitware.com>
Merge-request: !8292