Document window frame exclusions in fblangref40 and fblangref50

[mrotteveel]

Going over the code in parse.y, I noticed the presence of window frame exclusions (window_frame_exclusion_opt in parse.y), which were not documented in the Firebird 4.0 Release Notes, and thus also not in the Firebird 4.0 Language Reference.

Add this to the syntax of fblangref40 and fblangref50 and document its use.

[mrotteveel]

As it turns out, attempting to use any option other than EXCLUDE NO OTHERS will fail with a "feature is not supported; window EXCLUDE clause" error code 335544343 (isc_wish_list), so documenting this is not really relevant.