kconfig: update kconfig-language text

Add kconfig-language docs for mainmenu, def_bool, and def_tristate.
Remove "requires" as a synonym of "depends on" since it was removed
from  the parser in commit 247537b9a270b52cc0375adcb0fb2605a160cba5.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Documentation/kbuild/kconfig-language.txt

@@ -77,7 +77,12 @@ applicable everywhere (see syntax).
   Optionally, dependencies only for this default value can be added with
   "if".
 
-- dependencies: "depends on"/"requires" <expr>
+- type definition + default value:
+	"def_bool"/"def_tristate" <expr> ["if" <expr>]
+  This is a shorthand notation for a type definition plus a value.
+  Optionally dependencies for this default value can be added with "if".
+
+- dependencies: "depends on" <expr>
   This defines a dependency for this menu entry. If multiple
   dependencies are defined, they are connected with '&&'. Dependencies
   are applied to all other options within this menu entry (which also
@@ -289,3 +294,10 @@ source:
 	"source" <prompt>
 
 This reads the specified configuration file. This file is always parsed.
+
+mainmenu:
+
+	"mainmenu" <prompt>
+
+This sets the config program's title bar if the config program chooses
+to use it.