Documentation: kconfig: explicitly document missing prompt

There are a few lines in the kbuild-language.rst document which
obliquely reference the behavior of config options without prompts.
But there is nothing in the obvious location that explicitly calls
out that users cannot edit config options unless they have a prompt.

Signed-off-by: Stephen Brennan <[e-mail]>
Reviewed-by: Nathan Chancellor <[e-mail]>
Signed-off-by: Masahiro Yamada <[e-mail]>

Documentation/kbuild/kconfig-language.rst

···
 
   Every menu entry can have at most one prompt, which is used to display
   to the user. Optionally dependencies only for this prompt can be added
-  with "if".
+  with "if". If a prompt is not present, the config option is a non-visible
+  symbol, meaning its value cannot be directly changed by the user (such as
+  altering the value in ``.config``) and the option will not appear in any
+  config menus. Its value can only be set via "default" and "select" (see
+  below).
 
 - default value: "default" <expr> ["if" <expr>]