Christian Couder [off-list ref] writes:

quoted

+This command can also operate on the output of  
linkgit:git-format-patch[1],
+which is more elaborate than a plain commit message. Namely, such output
+includes a commit message (as above), a "---" divider line, and a patch  
part.
+For these inputs, the divider and patch parts are ignored, unless  
`--no-divider`
+is specified.

I think saying "the divider and patch parts are ignored" is a bit
ambiguous. It could mean that when a patch is read by the command only
its commit message, possibly modified by the command, is emitted on
the standard output.

Indeed. Updated in v4.