Improve some `wp-admin/includes/class-wp-filesystem-*.php` docs for `@param`.

See #30799.

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Improve some docs for `@param`. Remove an unneeded `$wpdb` global import.

See #30799.

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In `wp_import_handle_upload()`:

`$file` was essentially getting declared/overwritten 3 times. In lieu of this, return an array containing the error immediately instead of doing a short-circuit array key assignment on error. Rename the local variable to `$upload` and use its properties instead of creating 3 new local vars, one of which stomped the array.

See #30799.

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In `image_size_input_fields()`:

* Declare `$out` as an empty array - this is not a strict PHP requirement, but is a good practice before loops
* Most of this function was tabbed twice, instead of once

See #30799.

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For clarity, initialize some arrays that previously were only assigned via short circuit in loops.

See #30799.

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Correct the `@param` docs for arguments that are truthy/falsey.

See #30799.

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git-svn-id: http://core.svn.wordpress.org/trunk@30969 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Comment List Tables:

* Declare `$extra_items` property
* Since `->get_column_info()` overrides its parent's method, it doesn't need to set `_column_headers`, which is an undeclared property that is only used for caching the lookup after the first run in the parent class, which isn't called in the child class.
* `$_args` in `WP_List_Table` has to be `protected` for `WP_Post_Comments_List_Table` to legally access it. `$_args` is awkward as is and should probably be only available via a getter.

See #30799.

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