Help: drop confusing reference to `if()` from `$<BOOL:>`

`if()` uses the internal `IsOn` call whild `$<BOOL:>` uses `IsOff`. For
historical reasons, these are not inverses of each other, so drop
mention of `if()` from the documentation of `$<BOOL:>`.

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

@@ -47,8 +47,8 @@ Logical Operators
 -----------------
 
 ``$<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:
+  Converts ``string`` to ``0`` or ``1``. Evaluates to ``0`` if any of the
+  following is true:
 
   * ``string`` is empty,
   * ``string`` is a case-insensitive equal of