Correct zfs-send(8) on readonly sends

zfs-send(8) claimed in the flags list you could use -pR when sending
a readonly filesystem or volume. You cannot.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #12336

man/man8/zfs-send.8

@@ -44,7 +44,7 @@
 .Ar snapshot
 .Nm zfs
 .Cm send
-.Op Fl DLPRcenpsvw
+.Op Fl DLPcensvw
 .Op Fl i Ar snapshot Ns | Ns Ar bookmark
 .Ar filesystem Ns | Ns Ar volume Ns | Ns Ar snapshot
 .Nm zfs
@@ -285,7 +285,7 @@ You will be able to receive your streams on future versions of ZFS.
 .It Xo
 .Nm zfs
 .Cm send
-.Op Fl DLPRcenpvw
+.Op Fl DLPcenvw
 .Op Fl i Ar snapshot Ns | Ns Ar bookmark
 .Ar filesystem Ns | Ns Ar volume Ns | Ns Ar snapshot
 .Xc
@@ -296,7 +296,11 @@ filesystem must not be mounted.
 When the stream generated from a filesystem or volume is received, the default
 snapshot name will be
 .Qq --head-- .
-.Bl -tag -width "-L"
+.Bl -tag -width "-D"
+.It Fl D , -dedup
+Deduplicated send is no longer supported.
+This flag is accepted for backwards compatibility, but a regular,
+non-deduplicated stream will be generated.
 .It Fl L , -large-block
 Generate a stream which may contain blocks larger than 128KB.
 This flag has no effect if the