Don't mention label abbreviation in search documentation

The search documentation says that a label may be specified by
its one or two character abbreviation.

This is no longer the supported; remove it from the documentation.

Bug: Issue 2207
Change-Id: I8e81d244110b17e142e50bdd0ade8f9dfa3180d5
diff --git a/Documentation/user-search.txt b/Documentation/user-search.txt
index c40786b..7c4ca52 100644
--- a/Documentation/user-search.txt
+++ b/Documentation/user-search.txt
@@ -326,9 +326,6 @@
 
 * The label name.  Example: `label:Code-Review`.
 
-* The one or two character abbreviation shown in the column header
-  of change list pages.  Example: `label:R` or `label:V`.
-
 * The label name followed by a ',' followed by a reviewer id or a
   group id.  To make it clear whether a user or group is being looked
   for, precede the value by a user or group argument identifier