patch 8.1.2231: not easy to move to the middle of a text line

Problem:    Not easy to move to the middle of a text line.
Solution:   Add the gM command. (Yasuhiro Matsumoto, closes #2070)
diff --git a/runtime/doc/motion.txt b/runtime/doc/motion.txt
index f0935da..9ac460a 100644
--- a/runtime/doc/motion.txt
+++ b/runtime/doc/motion.txt
@@ -1,4 +1,4 @@
-*motion.txt*    For Vim version 8.1.  Last change: 2019 Jun 02
+*motion.txt*    For Vim version 8.1.  Last change: 2019 Oct 28
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -227,6 +227,12 @@
 gm			Like "g0", but half a screenwidth to the right (or as
 			much as possible).
 
+							*gm* *gM*
+gM			Like "g0", but to halfway the text of the line.
+			With a count: to this percentage of text in the line.
+			Thus "10gM" is near the start of the text and "90gM"
+			is near the end of the text.
+
 							*g$* *g<End>*
 g$ or g<End>		When lines wrap ('wrap' on): To the last character of
 			the screen line and [count - 1] screen lines downward