Better to be explicit I suppose.
How about:
-In the absence of this option, each destination file is created with the
+In the absence of this option, the permissions of existing destination
+files are unchanged, while each new file is created with the
mode bits of the corresponding source file, minus the bits set in the
umask and minus the set-user-ID and set-group-ID bits.
@xref{File permissions}.thanks, Pádraig.
