Add: Block specific toolbar button sample to the format api tutorial (#…

…14113)

## Description
This PR updates the format API tutorial (toolbar button section) to include a sample of a button that only renders on a certain block.
Answers a question posted on #14104.
Closes: #14104

## How has this been tested?
I pasted the sample code on the browser console and verified it works as expected.

docs/designers-developers/developers/tutorials/format-api/2-toolbar-button.md

@@ -35,3 +35,54 @@ Let's check that everything is working as expected. Reload the post/page and sel
 ![Toolbar with custom button](https://raw.githubusercontent.com/WordPress/gutenberg/master/docs/designers-developers/assets/toolbar-with-custom-button.png)
 
 You may also want to check that upon clicking the button the `toggle format` message is shown in your browser's console.
+
+## Show the button only for specific blocks
+
+By default, the button is rendered on every rich text toolbar (image captions, buttons, paragraphs, etc).
+It is possible to render the button only on blocks of a certain type by using `wp.data.withSelect` together with `wp.compose.ifCondition`.
+The following sample code renders the previously shown button only on paragraph blocks:
+
+```js
+( function( wp ) {
+	var withSelect = wp.data.withSelect;
+	var ifCondition = wp.compose.ifCondition;
+	var compose = wp.compose.compose;
+	var MyCustomButton = function( props ) {
+		return wp.element.createElement(
+			wp.editor.RichTextToolbarButton, {
+				icon: 'editor-code',
+				title: 'Sample output',
+				onClick: function() {
+					console.log( 'toggle format' );
+				},
+			}
+		);
+	}
+	var ConditionalButton = compose(
+		withSelect( function( select ) {
+			return {
+				selectedBlock: select( 'core/editor' ).getSelectedBlock()
+			}
+		} ),
+		ifCondition( function( props ) {
+			return (
+				props.selectedBlock &&
+				props.selectedBlock.name === 'core/paragraph'
+			); 
+		} )
+	)( MyCustomButton );
+
+	wp.richText.registerFormatType(
+		'my-custom-format/sample-output', {
+			title: 'Sample output',
+			tagName: 'samp',
+			className: null,
+			edit: ConditionalButton,
+		}
+	);
+} )( window.wp );
+```
+
+Don't forget adding `wp-compose` and `wp-data` to the dependencies array in the PHP script.
+
+More advanced conditions can be used, e.g., only render the button depending on specific attributes of the block.