Help: Document add_custom_command PRE_BUILD/PRE_LINK for exe/lib only (#15059)

State explicitly that the PRE_LINK mode is not for targets created by
the add_custom_target command.  The existing wording for PRE_BUILD being
treated as PRE_LINK by non-VS generators will now imply this restriction
for PRE_BUILD too.

Help/command/add_custom_command.rst

@@ -159,5 +159,7 @@ of the following is specified:
 ``PRE_LINK``
   Run after sources have been compiled but before linking the binary
   or running the librarian or archiver tool of a static library.
+  This is not defined for targets created by the
+  :command:`add_custom_target` command.
 ``POST_BUILD``
   Run after all other rules within the target have been executed.