bug#51961: image-mode: advertize viewing an image as hex less eagerly

[Stefan Kangas <stefan@marxist.se>]

Severity: minor

This is a usability bug:

I don't think image mode should advertize it's capability to view an
image as hex in a message every time it is enabled:

    Type C-c C-c or C-c C-x to view the image as text or hex

Editing the hex or "text" of something like a JPEG does not strike me as
something most people will be able to do usefully by hand.  For bitmap
formats maybe, but even then, won't people just use an image editor?

And for those users that do know what bits to flip, can't we expect them
to be able to figure out that Emacs has a command to quickly flip to the
built-in hex editor or to "text mode"?  For example by reading the mode
help?  IOW, I think this message confuses more than it helps (but see
the final paragraph below).

Why not advertize some useful commands instead, like "Press h for help",
or "Use + and - to scale up and down"?

Or, even better perhaps, just say nothing?  It's already all there in
`describe-mode', right?

There is a minority of image types where it is obviously useful,
i.e. XPM and similar formats where the image is actually just a plain
text file.  I think we could consider keeping the message for those.