docs: Reword `str::strip_prefix` and `strip_suffix` a bit
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 <d68d4287c1430ca128ad85033356c9d259238c74@chiark.greenend.org.uk>

graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust