patch 8.2.3226: new digraph functions use old naming scheme

Problem:    New digraph functions use old naming scheme.
Solution:   Use the digraph_ prefix. (Hirohito Higashi, closes #8580)
diff --git a/runtime/doc/usr_41.txt b/runtime/doc/usr_41.txt
index df9ebe6..2651bb8 100644
--- a/runtime/doc/usr_41.txt
+++ b/runtime/doc/usr_41.txt
@@ -994,16 +994,16 @@
 	winrestview()		restore saved view of current window
 
 Mappings and Menus:			    *mapping-functions*
+	digraph_get()		get |digraph|
+	digraph_getlist()	get all |digraph|s
+	digraph_set()		register |digraph|
+	digraph_setlist()	register multiple |digraph|s
 	hasmapto()		check if a mapping exists
 	mapcheck()		check if a matching mapping exists
 	maparg()		get rhs of a mapping
 	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