Docs: Add missing summaries and `@since` versions to DocBlocks for the...

Docs: Add missing summaries and `@since` versions to DocBlocks for the `_get_display_callback()`, `_get_update_callback()`, and `_get_form_callback()` methods in `WP_Widget`.

Introduced in [10764].

See #32246.

Built from https://develop.svn.wordpress.org/trunk@35955


git-svn-id: https://core.svn.wordpress.org/trunk@35919 1a063a9b-81f0-0310-95a4-ce76da25c4cd