Help: Document the purpose of usage requirements clearly.

People will be tempted to put things there for convenience, thereby
causing conflicts similar to

 http://thread.gmane.org/gmane.comp.compilers.clang.devel/35162/focus=35169

where it is conceivable that the LLVM developers could put a flag on
a target for convenience, which would cause conflicts for some downstreams.

Help/manual/cmake-buildsystem.7.rst

@@ -112,6 +112,12 @@ Each command may be invoked with multiple uses of each keyword:
     INTERFACE USING_ARCHIVE_LIB
   )
 
+Note that usage requirements are not designed as a way to make downstreams
+use particular :prop_tgt:`COMPILE_OPTIONS` or
+:prop_tgt:`COMPILE_DEFINITIONS` etc for convenience only.  The contents of
+the properties must be **requirements**, not merely recommendations or
+convenience.
+
 Target Properties
 -----------------