Help: improve details on `CMAKE_LANG_CREATE_SHARED_*`

Add a note that the `CMAKE_<LANG>_CREATE_SHARED_*` rule variables are
semicolon delimited lists that can run multiple commands.

Help/variable/CMAKE_LANG_CREATE_SHARED_LIBRARY.rst

@@ -4,4 +4,5 @@ CMAKE_<LANG>_CREATE_SHARED_LIBRARY
 Rule variable to create a shared library.
 
 This is a rule variable that tells CMake how to create a shared
-library for the language ``<LANG>``.
+library for the language ``<LANG>``.  This rule variable is a ``;`` delimited
+list of commands to run to perform the linking step.

Help/variable/CMAKE_LANG_CREATE_SHARED_MODULE.rst

@@ -4,4 +4,5 @@ CMAKE_<LANG>_CREATE_SHARED_MODULE
 Rule variable to create a shared module.
 
 This is a rule variable that tells CMake how to create a shared
-library for the language ``<LANG>``.
+library for the language ``<LANG>``.  This rule variable is a ``;`` delimited
+list of commands to run.