docs: Reword `slice::strip_prefix` and `strip_suffix` a bit

The stabilisation issue, #73413, has an open item for documentation.
I looked at the docs and it is all there, but I felt it could do with
some minor wording improvement.

I looked at the `str::strip_prefix` docs for a template.  (That
resulted in me slightly changing that doc too.)

I de-linkified `None` and `Some`, as I felt that rather noisy..  I
searched stdlib, and these don't seem to be usually linkified.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

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