patch 9.0.0438: cannot put virtual text above a line

Problem:    Cannot put virtual text above a line.
Solution:   Add the "above" value for "text_align".
diff --git a/runtime/doc/textprop.txt b/runtime/doc/textprop.txt
index 996cdd1..8d7c0ec 100644
--- a/runtime/doc/textprop.txt
+++ b/runtime/doc/textprop.txt
@@ -153,6 +153,7 @@
 					   the text wraps to the next screen
 					   line)
 				   below   in the next screen line
+				   above   just above the line
 				When omitted "after" is used.  Only one
 				"right" property can fit in each line, if
 				there are two ore more these will go in a