Twenty Nineteen: Improve comments in `twentynineteen_setup`

[laurelfulford]

Originally reported by @joyously in the Twenty Nineteen GitHub repo:

The new interfaces for the editor should be clear and instructive. Many people will look to this theme for "How To" information.

The comments need to indicate what the add_theme_support really does. The call for the old editor should be marked as such.

Currently, it's a bit sparse:

// Add support for Block Styles.
add_theme_support( 'wp-block-styles' );

// Add support for full and wide align images.
add_theme_support( 'align-wide' );

// Add support for editor styles.
add_theme_support( 'editor-styles' );

// Enqueue editor styles.
add_editor_style( 'style-editor.css' );

// Editor color palette.
add_theme_support(
	'editor-color-palette',
		array(
			array(
				'name'  => __( 'Primary Color', 'twentynineteen' ),
					'slug'  => 'primary',
				'color' => twentynineteen_hsl_hex( 'default' === get_theme_mod( 'primary_color' ) ? 199 : get_theme_mod( 'primary_color_hue', 199 ), 100, 33 ),
				),
			)
		);

// Add support for responsive embedded content.
add_theme_support( 'responsive-embeds' );

There is also a separate function for enqueuing twentynineteen-editor-customizer-styles which should have good comments. This handle name is questionable, because it is editor and customizer, so it's confusing. Does it need to say customizer?

If I'm looking at this to figure out how to change my theme for the new editor, I can't really tell what's what.

Copied over from: ​https://github.com/WordPress/twentynineteen/issues/634

[sabernhardt]

These comments are the same in Twenty Twenty and Twenty Twenty-One.

Here are suggestions for the first three theme supports:

	// Add support for block-specific styles on the front end.
	add_theme_support( 'wp-block-styles' );

	// Add support for full and wide align images in blocks.
	add_theme_support( 'align-wide' );

	// Add support for editor styles in both classic and block editors.
	add_theme_support( 'editor-styles' );