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

Brad Kingrequested to merge
brad.king/cmake:doc-if-AND-OR into master

The wording update in commit b74819e4 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.

Backport: release
Fixes: #23207 (closed)

Edited by Brad King

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