add docs for argparse.boolean

added entry to changelog as well.

docs/changelog.txt

@@ -64,6 +64,7 @@ Template for new versions:
 ## API
 
 ## Lua
+- ``argparse.boolean``: convert arguments to lua boolean values.
 
 ## Removed
 

docs/dev/Lua API.rst

@@ -3538,6 +3538,12 @@ parameters.
   ``tonumber(arg)``. If ``arg_name`` is specified, it is used to make error
   messages more useful.
 
+* ``argparse.boolean(arg, arg_name)``
+  Converts ``string.lower(arg)`` from "yes/no/on/off/true/false/etc..." to a lua
+  boolean. Throws if the value can't be converted, otherwise returns
+  ``true``/``false``. If ``arg_name`` is specified, it is used to make error
+  messages more useful.
+
 dumper
 ======