kconfig: Document the new "visible if" syntax

Signed-off-by: Michal Marek <mmarek@suse.cz>

Documentation/kbuild/kconfig-language.txt

@@ -113,6 +113,13 @@ applicable everywhere (see syntax).
 	That will limit the usefulness but on the other hand avoid
 	the illegal configurations all over.
 
+- limiting menu display: "visible if" <expr>
+  This attribute is only applicable to menu blocks, if the condition is
+  false, the menu block is not displayed to the user (the symbols
+  contained there can still be selected by other symbols, though). It is
+  similar to a conditional "prompt" attribude for individual menu
+  entries. Default value of "visible" is true.
+
 - numerical ranges: "range" <symbol> <symbol> ["if" <expr>]
   This allows to limit the range of possible input values for int
   and hex symbols. The user can only input a value which is larger than
@@ -303,7 +310,8 @@ menu:
 	"endmenu"
 
 This defines a menu block, see "Menu structure" above for more
-information. The only possible options are dependencies.
+information. The only possible options are dependencies and "visible"
+attributes.
 
 if: