Add doc for isFullySelected

editor/utils/dom.js

@@ -342,6 +342,14 @@ export function documentHasSelection() {
 	return range && ! range.collapsed;
 }
 
+/**
+ * Check wether the contents of the element have been fully selected.
+ * Returns true if there is no possibility of full selection.
+ *
+ * @param {Element} element The element to check.
+ *
+ * @returns {Boolean} True if fully selected, false if not.
+ */
 export function isFullySelected( element ) {
 	if ( includes( [ 'INPUT', 'TEXTAREA' ], element.nodeName ) ) {
 		return element.selectionStart === 0 && element.value.length === element.selectionEnd;