doc: update isUtf8 description

Author: anonrig

doc/api/buffer.md

@@ -5137,9 +5137,12 @@ added: REPLACEME
 -->
 
 * input {Buffer | ArrayBuffer | TypedArray} The input to validate.
-* Returns: {boolean} Returns `true` if and only if the input is valid UTF-8.
+* Returns: {boolean}
+
+This function is used to check if input contains invalid UTF-8 code points (characters).
+Returns `true` if and only if the input does not contain non UTF-8 characters.
 
-This function is used to check if input contains UTF-8 code points (characters).
+Throws if the `input` is a detached array buffer.
 
 ### `buffer.INSPECT_MAX_BYTES`