update squash description in the new/edit merge request form

app/views/shared/issuable/form/_branch_chooser.html.haml

@@ -35,5 +35,5 @@
       .checkbox
         = label_tag 'merge_request[squash]' do
           = check_box_tag 'merge_request[squash]', nil
-          Squash commits on merge. The commit message for the last commit will
-          be used.
+          Squash commits when merge request is accepted.
+          = link_to 'Learn more', help_page_path('user/project/merge_requests/squash_before_merge.md')

doc/user/project/merge_requests/squash_before_merge.md

+# Squash before merge
+
+Documentation forthcoming.