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)
diff --git a/runtime/doc/digraph.txt b/runtime/doc/digraph.txt
index 79c7cf0..9715a88 100644
--- a/runtime/doc/digraph.txt
+++ b/runtime/doc/digraph.txt
@@ -38,6 +38,9 @@
 <			Avoid defining a digraph with '_' (underscore) as the
 			first character, it has a special meaning in the
 			future.
+			NOTE: This command cannot add a digraph that starts
+			with a white space.  If you want to add such digraph,
+			you can use |setdigraph()| instead.
 
 Vim is normally compiled with the |+digraphs| feature.  If the feature is
 disabled, the ":digraph" command will display an error message.