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

"Some is returned with <some value>" is an awkward construction.
The use of the passive voice is a bit odd, and doesn't seem like the
house style.

So say instead "returns X, wrapped in `Some`", for which there is some
other precedent in stdlib.

Instead of repeating "with the prefix removed", say "after the
prefix".  This is a bit clearer that the original is not modified.

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

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