Define "character" in str.slice docstring

[mcrumiller]

Problem description

From #10339, the docstring for str.slice currently is:

        Create subslices of the string values of a Utf8 Series.

        Parameters
        ----------
        offset
            Start index. Negative indexing is supported.
        length
            Length of the slice. If set to ``None`` (default), the slice is taken to the
            end of the string.

It's a bit ambiguous as to whether or not a slice returns bytes or characters/codepoints. Using the following:

>>> import polars as pl

>>> # the following is Google's translation of "polar bear" into Arabic. It is 21 bytes with 11 code points.
>>> s = 'الدب القطبي'
>>> [len(c.encode()) for c in s]
[2, 2, 2, 2, 1, 2, 2, 2, 2, 2, 2]

Polars slices return codepoints:

>>> s2 = pl.Series([s])
>>> len(s2.str.slice(0, 1)[0].encode())
2

We should probably specify this. I think I'll go by @avimallu's recommendation.

[orlp]

For reference, this is what Rust's str::chars method has as its documentation:

As a string slice consists of valid UTF-8, we can iterate through a string slice by char. This method returns such an iterator.

It’s important to remember that char represents a Unicode Scalar Value, and might not match your idea of what a ‘character’ is. Iteration over grapheme clusters may be what you actually want. This functionality is not provided by Rust’s standard library, check crates.io instead.

[mcrumiller]

@ritchie46 I ended up just doing small implementations since I wasn't sure where aliasing should stop; maybe the compiler can detect some shortcut this way. Looks like everything is working.