Update invalid block copy

Try and make it clearer while not wrapping any buttons in invalid messages

edit-post/hooks/validate-multiple-use/index.js

@@ -96,7 +96,7 @@ const withMultipleValidation = createHigherOrderComponent( ( BlockEdit ) => {
 				] }
 			>
 				<strong>{ blockType.title }: </strong>
-				{ __( 'This block may not be used more than once.' ) }
+				{ __( 'This block can only be used once.' ) }
 			</Warning>,
 		];
 	} );

packages/editor/src/components/block-list/block-invalid-warning.js

@@ -22,20 +22,19 @@ function BlockInvalidWarning( { convertToHTML, convertToBlocks, convertToClassic
 		<Warning
 			actions={ [
 				<Button key="convert" onClick={ convertToBlocks } isLarge isPrimary={ ! hasHTMLBlock }>
-					{ __( 'Convert to Blocks' ) }
+					{ __( 'Auto-convert' ) }
 				</Button>,
 				hasHTMLBlock && (
 					<Button key="edit" onClick={ convertToHTML } isLarge isPrimary>
-						{ __( 'Keep as HTML' ) }
+						{ __( 'Keep' ) }
 					</Button>
 				),
 			] }
 			secondaryActions={ [
-				{ title: __( 'Convert to Blocks' ), onClick: convertToBlocks },
 				{ title: __( 'Convert to Classic Block' ), onClick: convertToClassic },
 			] }
 		>
-			{ __( 'This block has been modified externally.' ) }
+			{ __( 'This block contains unexpected content.' ) }
 		</Warning>
 	);
 }