Help: Document MAIN_DEPENDENCY limitation in add_custom_command

Specify explicitly that at most one custom command may use a given
source file as its main dependency.

Help/command/add_custom_command.rst

@@ -130,7 +130,8 @@ The options are:
   Specify the primary input source file to the command.  This is
   treated just like any value given to the ``DEPENDS`` option
   but also suggests to Visual Studio generators where to hang
-  the custom command.
+  the custom command.  At most one custom command may specify a
+  given source file as its main dependency.
 
 ``OUTPUT``
   Specify the output files the command is expected to produce.