docs: add docs for abbr_length and menu_length

doc/completion-nvim.txt

@@ -369,5 +369,20 @@ g:completion_sorting				    *g:completion_sorting*
 
 	default value: 'alphabet'
 
+g:completion_abbr_length			*g:completion_abbr_length*
+
+	Some language server have long snippets items, which can make your
+	completion menu super long. This option enable you to truncate
+	item.abbr with a maximum length.
+
+	default value: 0(which means no truncates)
+
+g:completion_menu_length			*g:completion_menu_length*
+
+	Similar to `g:completion_abbr_length`, language server may populate
+	the completion menus with long menu items. This option enable you
+	truncate item.menu with a maximum length.
+
+	default value: 0(which means no truncates)
 ==============================================================================
 vim:tw=78:ts=8:ft=help:norl:noet:fen:noet: