Documentation: avoid "uninteresting"

The documentation of --boundary uses the term "uninteresting", which is
not used or defined anywhere else in the documentation. This is
unhelpful and confusing to anyone who hasn't seen the UNINTERESTING
flag in the source code.

Change to use "excluded", as per revisions.txt.

Signed-off-by: Kevin Bracey <kevin@bracey.fi>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/rev-list-options.txt

@@ -271,8 +271,8 @@ See also linkgit:git-reflog[1].
 
 --boundary::
 
-	Output uninteresting commits at the boundary, which are usually
-	not shown.
+	Output excluded boundary commits. Boundary commits are
+	prefixed with `-`.
 
 --