patch 8.1.2225: the "last used" info of a buffer is under used

Problem:    The "last used" info of a buffer is under used.
Solution:   Add "lastused" to getbufinfo(). List buffers sorted by last-used
            field. (Andi Massimino, closes #4722)
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index 029a072..6e4ff1d 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -8689,6 +8689,8 @@
 			complete first match.
 	"list:longest"	When more than one match, list all matches and
 			complete till longest common string.
+	"list:lastused" When more than one buffer matches, sort buffers
+			by time last used (other than the current buffer).
 	When there is only a single match, it is fully completed in all cases.
 
 	Examples: >