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/eval.txt b/runtime/doc/eval.txt
index a167d5f..f50c8c5 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -4778,6 +4778,10 @@
 			changed		TRUE if the buffer is modified.
 			changedtick	number of changes made to the buffer.
 			hidden		TRUE if the buffer is hidden.
+			lastused	timestamp in seconds, like
+					|localtime()|, when the buffer was
+					last used.
+					{only with the |+viminfo| feature}
 			listed		TRUE if the buffer is listed.
 			lnum		current line number in buffer.
 			loaded		TRUE if the buffer is loaded.