Correct Edit documentation
Correct Edit documentation

There were missing doc comments for a few basic properties, + the text about `edit_text` property being a read-only property was a lie.
tonycpsu/urwid,urwid/urwid,wardi/urwid,rndusr/urwid,inducer/urwid,wardi/urwid,tonycpsu/urwid,rndusr/urwid,urwid/urwid,inducer/urwid,tonycpsu/urwid,urwid/urwid,rndusr/urwid,inducer/urwid,wardi/urwid