Clarify that the change/checksum itemize flag can be missing

when talking to older rsync versions.

rsync.yo

@@ -1677,6 +1677,8 @@ quote(itemization(
   it() A bf(c) means either that a regular file has a different checksum
   (requires bf(--checksum)) or that a symlink, device, or special file has
   a changed value.
+  Note that if you are sending files to an rsync prior to 3.0.1, this
+  change flag will be present for checksum-differing regular files.
   it() A bf(s) means the size of the file is different and will be updated
   by the file transfer.
   it() A bf(t) means the modification time is different and is being updated