patch 9.0.0738: cannot suppress completion "scanning" messages

Problem:    Cannot suppress completion "scanning" messages.
Solution:   Add the "C" flag in 'shortmess'. (Bjorn Linse, closes #11354)
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index b3c4a40..ee6e5da 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -7079,6 +7079,8 @@
 	  c	don't give |ins-completion-menu| messages.  For example,
 		"-- XXX completion (YYY)", "match 1 of 2", "The only match",
 		"Pattern not found", "Back at original", etc.
+	  C	don't give messages while scanning for ins-completion items,
+		for instance "scanning tags"
 	  q	use "recording" instead of "recording @a"
 	  F	don't give the file info when editing a file, like `:silent`
 		was used for the command; note that this also affects messages