Recent Posts Widget: Use `ob_end_flush()` instead of `ob_flush()`.

`ob_end_flush()` flushes the output buffer *and* turns output buffering off, same as `ob_get_flush()`.

props m_i_n.
see #28009 for trunk.

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Force users to choose a nav menu in the custom nav menu widget, for a better customizer UX

Before, they had to make a dummy change to get it to render. Now they
are made to choose a nav menu from the dropdown, which feels more
natural.

fixes #27878 for trunk. props westonruter
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Use '&mdash; Select &mdash;' instead of '-- Select --' for nav menus. Looks nicer.

see #27878
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git-svn-id: http://core.svn.wordpress.org/trunk@28035 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Ensure the `nav_menu_meta_box_object` filter hook is only documented once.

See #26869.

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Ensure the `register` filter hook is only documented once.

See #26869.

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Ensure the `style_loader_tag` filter hook is only documented once.

See #26869.

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Ensure the `submitlink_box` action hook is only documented once.

See #26869.

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Ensure adjacency of a duplicate hook comment for a `the_category` filter call.

See #26869.

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git-svn-id: http://core.svn.wordpress.org/trunk@28041 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Ensure all duplicate calls of the `wp_create_file_in_uploads` filter are labeled as such.

See #26869.

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Ensure the `wp_edit_nav_menu_walker` filter is only documented once.

See #26869.

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Ensure the `wp_set_comment_status` action hook is only documented once.

See #26869.

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git-svn-id: http://core.svn.wordpress.org/trunk@28044 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Clean up duplicate hook notations and adjacency for calls to the `wp_signup_location` filter.

Also adds braces missed in [25535].

See #26869.

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Mark the `$hook` and `$page_hook` hooks as `@internal` to skip parsing.

See #26869.

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