Added a description for the embed block - revised for Gutenberg from …

…the codex embed description found here: https://codex.wordpress.org/Embeds

Resolves WordPress#2833

blocks/library/embed/index.js

@@ -147,9 +147,18 @@ function getEmbedBlockSettings( { title, icon, category = 'embed', transforms, k
 					)
 				);
 
+				const inspectorControls = focus && (
+					<InspectorControls key="inspector">
+						<BlockDescription>
+							<p>{ __( 'The Embed block allows you to easily add videos, images, tweets, audio, and other content to your content.' ) }</p>
+						</BlockDescription>
+					</InspectorControls>
+				);
+
 				if ( fetching ) {
 					return [
 						controls,
+						inspectorControls,
 						<div key="loading" className="wp-block-embed is-loading">
 							<Spinner />
 							<p>{ __( 'Embedding…' ) }</p>
@@ -162,6 +171,7 @@ function getEmbedBlockSettings( { title, icon, category = 'embed', transforms, k
 
 					return [
 						controls,
+						inspectorControls,
 						<Placeholder key="placeholder" icon={ icon } label={ label } className="wp-block-embed">
 							<form onSubmit={ this.doServerSideRender }>
 								<input
@@ -192,6 +202,7 @@ function getEmbedBlockSettings( { title, icon, category = 'embed', transforms, k
 
 				return [
 					controls,
+					inspectorControls,
 					<figure key="embed" className={ typeClassName }>
 						{ ( cannotPreview ) ? (
 							<Placeholder icon={ icon } label={ __( 'Embed URL' ) }>