patch 8.2.3184: cannot add a digraph with a leading space

Problem:    Cannot add a digraph with a leading space.  It is not easy to list
            existing digraphs.
Solution:   Add setdigraph(), setdigraphlist(), getdigraph() and
            getdigraphlist(). (closes #8580)

runtime/doc/usr_41.txt

@@ -1000,6 +1000,10 @@ Mappings and Menus:			    *mapping-functions*
 	mapset()		restore a mapping
 	menu_info()		get information about a menu item
 	wildmenumode()		check if the wildmode is active
+	getdigraph()		get |digraph|
+	getdigraphlist()	get all |digraph|s
+	setdigraph()		register |digraph|
+	setdigraphlist()	register multiple |digraph|s
 
 Testing:				    *test-functions*
 	assert_equal()		assert that two expressions values are equal