DOC: tweaks and fixes for string functionality docs - #32755

PR summary

Fixes a number of minor issues and inaccuracies in the docs.

• Using r"" strings in the docstrings for encode and decode is necessary because there are inline control characters.
• Refer to null-padded strings instead of zero-padded or null-terminated strings
• Correct docs that predate StringDType short string support
• Don't talk about raw bytes as an encoding.

AI Disclosure

An AI helped me spot these problems as part of work on bytestringdtype.