Update example of str.split, bytes.split (#121287)

In `{str,bytes}.strip(chars)`, multiple characters are not treated as a
prefix/suffix, but as individual characters. This may make users confuse
whether `split` has similar behavior.
Users may incorrectly expect that
`'Good morning, John.'.split(', .') == ['Good', 'morning', 'John']`

Adding a bit of clarification in the doc.

Co-authored-by: Yuxin Wu <ppwwyyxx@users.noreply.github.com>