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/windows.txt b/runtime/doc/windows.txt
index 0c37cf5..813de1b 100644
--- a/runtime/doc/windows.txt
+++ b/runtime/doc/windows.txt
@@ -1090,6 +1090,7 @@
 		     R	 terminal buffers with a running job
 		     F	 terminal buffers with a finished job
 		     ?   terminal buffers without a job: `:terminal NONE`
+		     t   show time last used and sort buffers
 		Combining flags means they are "and"ed together, e.g.:
 		     h+   hidden buffers which are modified
 		     a+   active buffers which are modified