Rich text: add comment on placeholder approach (#64945)

Author: ellatrix

packages/rich-text/src/to-tree.js

@@ -299,6 +299,8 @@ export function toTree( {
 		if ( shouldInsertPadding && i === text.length ) {
 			append( getParent( pointer ), ZWNBSP );
 
+			// We CANNOT use CSS to add a placeholder with pseudo elements on
+			// the main block wrappers because that could clash with theme CSS.
 			if ( placeholder && text.length === 0 ) {
 				append( getParent( pointer ), {
 					type: 'span',