add_definitions: Don't document genex support.

This command does not support generator expressions. The documentation
was mistakenly extended to claim it in commit v3.0.0-rc1~60^2~3 (Help: Mark
up the buildsystem commands documentation, 2014-02-03).

Help/command/add_definitions.rst

@@ -21,7 +21,5 @@ backwards compatibility.  See documentation of the
 properties for details on adding preprocessor definitions to specific
 scopes and configurations.
 
-Arguments to ``add_definitions`` may use "generator expressions" with
-the syntax "$<...>".  See the :manual:`cmake-generator-expressions(7)`
-manual for available expressions.  See the :manual:`cmake-buildsystem(7)`
-manual for more on defining buildsystem properties.
+See the :manual:`cmake-buildsystem(7)` manual for more on defining
+buildsystem properties.