Help: Explain conversion rules of $<BOOL:string>.

This resolves #18549.

Help/manual/cmake-generator-expressions.7.rst

@@ -44,8 +44,16 @@ Available boolean expressions are:
 Logical Operators
 -----------------
 
-``$<BOOL:condition>``
-  ``1`` if the ``condition`` is true, else ``0``
+``$<BOOL:string>``
+  Converts ``string`` to ``0`` or ``1`` according to the rules of the
+  :command:`if()` command.  Evaluates to ``0`` if any of the following is true:
+
+  * ``string`` is empty,
+  * ``string`` is a case-insensitive equal of
+    ``0``, ``FALSE``, ``OFF``, ``N``, ``NO``, ``IGNORE``, or ``NOTFOUND``, or
+  * ``string`` ends in the suffix ``-NOTFOUND`` (case-sensitive).
+
+  Otherwise evaluates to ``1``.
 
 ``$<AND:conditions>``
   where ``conditions`` is a comma-separated list of boolean expressions.