bug#22296: 25.1.50; Document changes to `delete-selection-mode'

[Drew Adams <drew.adams@oracle.com>]

> I looked at delsel.el, and the only user-visible change in behavior
> since v24.3 is the one that was already announced in NEWS.24:
> 
>   ** Delete Selection mode can now be used without Transient Mark mode.

Somehow I didn't notice that mention.

> If you know about other changes (that are not bugfixes), please name
> them.

Frankly, I do not understand all of the changes I see in the code
since 24.3.  There are a boatload of them.  `diff' the Emacs 25
version against the 24.3 version.

1. It "can now be used without transient mark mode" is misleading.

You could always turn off t-m-mode (after turning d-s-mode on)
and still use d-s-mode.  (But you might have been surprised to see
stuff disappear because you didn't notice what was in the region.)

The changes for this seem to be: (1) d-s-mode does not automatically
turn on t-m-mode now, and (2) d-s-mode does NOT have any effect unless
you turn on t-m-mode temporarily.  IOW, d-s-mode works now only with
temporary t-m-mode; it no longer works with a non-active region.
(And yes, this is a good change.)

2. d-s-mode now supports rectangular regions (?).  Not clear
to me what this is about, from a quick glance at the code.
Maybe you will characterize this as a bug fix, but the behavior
seems to be changed, in any case.

3. Command `delete-selection-repeat-replace-region' was added.
It seems to be bound, so far, only in CUA mode (to `M-v').
Not real clear to me what this is about/for.

4. Apparently you can no longer specify `kill' as the d-s behavior:
(put 'some-command 'delete-selection 'kill).  Well, it's still
supported for now, but `kill' has been removed from the file-header
Commentary, and the code comments say that it is deprecated.

Why is this?  Where is this deprecation documented?  Why
shouldn't a user or a library still be able to override a
`delete-selection' setting for a given command, to specify
it as `kill'?  Not clear to me what this is about.

5. There is an additional, undocumented (why?) possible value
for `delete-selection', which you can apparently put on a
command's symbol: `delete-selection-uses-region-p'.  Please
consider documenting this (e.g., not just mentioning it in NEWS).

6. The default delete-selection behavior for multiple commands
has apparently changed.  For example, `open-line' now has the
default behavior and not the `kill' behavior.  Likewise, unloading
(`delsel-unload-function') deals with a different command list.

Those are a start - things I see that I wonder about.  You may
others, or you may understand these things better or differently.