Docs: Clarify that meta_query REGEXP values are raw MySQL patterns

[patrickcoombe]

Trac ticket: https://core.trac.wordpress.org/ticket/58180

When a meta_query clause uses 'compare' => 'REGEXP' (or 'NOT REGEXP' / 'RLIKE'), the value is passed directly to MySQL's REGEXP operator. This is not obvious from the DocBlock, and a common mistake is to wrap the pattern in PCRE-style delimiters (e.g. /^bar/), which MySQL treats as literal characters.

This adds a note to the $value parameter in the WP_Meta_Query DocBlock clarifying that for the REGEXP-family compares the value is used as a raw MySQL regular expression and should not be wrapped in delimiters such as /.

Documentation only; no behavioral change.

Props letraceursnork.
Fixes #58180.

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