Help: Clarify precedence of AND and OR in 'if' conditions

The wording update in commit b74819e4fe (Help: Format 'if' command
documentation, 2013-12-18, v3.0.0-rc1~227^2~1) incorrectly implied that
`AND` has higher precedence than `OR`.  Although this is common in many
languages, it has never been true in CMake's implementation.  Revise
the wording to clarify the precedence.

Add a test case demonstrating the order.

Fixes: #23207