bug#38578: Say how to save a message but keep editing

[積丹尼 Dan Jacobson]

On (info "(message) Sending") please mention the write command to simply
save the buffer, but not leave it.
Yes we know it is C-x C-s. But you need to explicitly say so.

C-c C-c         message-send-and-exit
C-c C-d         message-dont-send
are not enough.

P.S., one would think that editing the same message every few days, would keep
the same file number. But it doesn't. But OK that is a behind the scenes
implenentation detail.

Gnus v5.13
GNU Emacs 26.3 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.11)
 of 2019-09-23, modified by Debian