Docs: Add missing DocBlocks for `mb_substr()` and `_mb_substr()`.

Both will be ignored from parsing as and serve as compat functions for PHP's `mb_substr()`, which is not enabled by default.

Introduced in [17621].

See #32246.

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git-svn-id: https://core.svn.wordpress.org/trunk@35982 1a063a9b-81f0-0310-95a4-ce76da25c4cd