bug#11520: 24.1.50; delete-selection-mode conflicts with electric-pair-mode

[Stefan Monnier]

> Attached is a patch against trunk.

Thanks, I really appreciate the addition of docstrings.
it's got a few cosmetic problems, so I've installed it and then
installed a subsequent patch on top of it.

Here are the issues I noticed:
- a hook is a symbol whose value contains a list of functions (modulo
  a few subtleties).  The functions themselves are not hooks.
  E.g. delete-selection-pre-hook is not a hook, just a function that's
  typically added on a particular hook.
  And the symbol symbol is named with "-hook" (rather than with
  "-hooks") for normal hooks and "-functions" for abnormal hooks.
- symbols in docstrings are written `foo' rather than 'foo.
- docstrings normally don't end with a line-separator (i.e. the closing
  " should not be on its own line).
- there's no self-insert-iso in Emacs, so I'd rather not touch it.
- use the imperative for the first line of a docstring.
- `keep' is not a value handled specially by delete-selection-helper,
  so it is handled as a hook, luckily there's no function on this hook
  (an unbound symbol is treated by `run-hook' as a symbol bound to nil)
  so it ends up behaving like nil, which is indeed what we need; so the
  end behavior is correct, but only "by accident".


        Stefan