patch 8.0.0878: no completion for :mapclear

Problem:    No completion for :mapclear.
Solution:   Add completion (Nobuhiro Takasaki et al. closes #1943)
diff --git a/runtime/doc/map.txt b/runtime/doc/map.txt
index 239c791..983fb2d 100644
--- a/runtime/doc/map.txt
+++ b/runtime/doc/map.txt
@@ -1279,6 +1279,7 @@
 	-complete=highlight	highlight groups
 	-complete=history	:history suboptions
 	-complete=locale	locale names (as output of locale -a)
+	-complete=mapclear	buffer argument
 	-complete=mapping	mapping name
 	-complete=menu		menus
 	-complete=messages	|:messages| suboptions