patch 8.0.1386: cannot select modified buffers with getbufinfo()

Problem:    Cannot select modified buffers with getbufinfo().
Solution:   Add the "bufmodified" flag. (Yegappan Lakshmanan, closes #2431)
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 93e818a..3afd675 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -4173,6 +4173,7 @@
 		be specified in {dict}:
 			buflisted	include only listed buffers.
 			bufloaded	include only loaded buffers.
+			bufmodified	include only modified buffers.
 
 		Otherwise, {expr} specifies a particular buffer to return
 		information for.  For the use of {expr}, see |bufname()|