Docs: Add a better summary, missing `@since` version, parameter and return...

Docs: Add a better summary, missing `@since` version, parameter and return descriptions to the DocBlock for `_get_widget_id_base()`.

Introduced in [11484].

See #32246.

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git-svn-id: https://core.svn.wordpress.org/trunk@35957 1a063a9b-81f0-0310-95a4-ce76da25c4cd