kconfig: remove ---help--- from documentation

Since commit 84af7a6194 ("checkpatch: kconfig: prefer 'help' over
'---help---'"), scripts/checkpatch.pl warns the use of ---help---.

Kconfig still supports ---help---, but new code should avoid using it.
Let's stop advertising it in documentation.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Documentation/kbuild/kconfig-language.rst

@@ -196,14 +196,11 @@ applicable everywhere (see syntax).
   or equal to the first symbol and smaller than or equal to the second
   symbol.
 
-- help text: "help" or "---help---"
+- help text: "help"
 
   This defines a help text. The end of the help text is determined by
   the indentation level, this means it ends at the first line which has
   a smaller indentation than the first line of the help text.
-  "---help---" and "help" do not differ in behaviour, "---help---" is
-  used to help visually separate configuration logic from help within
-  the file as an aid to developers.
 
 - misc options: "option" <symbol>[=<value>]