Docs: Add missing @return tag to WP_Image_Editor_Imagick::remove_pdf_alpha_channel()

[csemazharul]

Trac ticket: https://core.trac.wordpress.org/ticket/65916

WP_Image_Editor_Imagick::remove_pdf_alpha_channel() returns a WP_Error if Imagick::setImageAlphaChannel() throws, but there's no @return tag in the DocBlock. Been like that since the method was added in changeset 56271 (Trac #39216).

/**
 * Removes PDF alpha after it's been read.
 *
 * @since 6.4.0
 */
protected function remove_pdf_alpha_channel() {
	...
	} catch ( Exception $e ) {
		return new WP_Error( 'pdf_alpha_process_failed', $e->getMessage() );
	}
}

Found it while looking at Trac #65817. PHPStan's return.missing rule didn't catch it because there's no declared return type, so it got skipped even though set_imagick_time_limit() in the same file is being fixed for that rule in #13082.

This just adds the tag. Nothing else changes, and PHPCS output for the file is the same before and after.

One other thing I ran into: the only caller, on line 372, throws the return value away, while pdf_load_source() and ->valid() right next to it are both checked. Not sure what the right call is there. Returning the error from load() would kill the PDF preview completely instead of just giving you one with a black background, which seems worse. Or the WP_Error could come out since nothing reads it. Left it alone for now, can do either if someone has a preference.

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