Formatting: Improve the docblock for `sanitize_title()`.

Since sanitize title is largely used for html attributes, or post slugs, make note that accents will be removed and that the string will be limited to alphanumeric characters, underscores, and dashes.

Fixes #33756.
Props ericlewis, swissspidy, wonderboymusic, psdtohtmlguru, DrewAPicture, SergeyBiryukov, sjmur, gma992, shulard, ebinnion, whyisjake.


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