Document new --append option.

rsync.yo

@@ -298,6 +298,7 @@ to the detailed description below for a complete description.  verb(
      --suffix=SUFFIX         backup suffix (default ~ w/o --backup-dir)
  -u, --update                skip files that are newer on the receiver
      --inplace               update destination files in-place
+     --append                append data onto shorter files
  -d, --dirs                  transfer directories without recursing
  -l, --links                 copy symlinks as symlinks
  -L, --copy-links            transform symlink into referent file/dir
@@ -557,6 +558,14 @@ should not use this option to update files that are in use.  Also note that
 rsync will be unable to update a file in-place that is not writable by the
 receiving user.
 
+dit(bf(--append)) This causes rsync to update a file by appending data onto
+the end of the file, which presumes that the data that already exists on
+the receiving side is identical with the start of the file on the sending
+side.  If that is not true, the file will fail the checksum test, and the
+resend will do a normal bf(--inplace) update to correct the mismatch.  Any
+file on the receiving side that is longer than a file on the sending side
+is skipped.  Implies bf(--inplace).
+
 dit(bf(-d, --dirs)) Tell the sending side to include any directories that
 are encountered.  Unlike bf(--recursive), a directory's contents are not copied
 unless the directory was specified on the command-line as either "." or a