Update labels-or-instructions.html (#3888)

closes #3887

clarifies that wcag is not recommending use of placeholder text, and
instead recommends accompanying text to provide the information (which
could then be part of label or description - but those details are not
applicable to this SC so not specified in the update)

This helps better match [technique
g89](https://www.w3.org/WAI/WCAG22/Techniques/general/G89.html) which
includes the date format expectation as part of the label text

---------

Co-authored-by: Patrick H. Lauke <redux@splintered.co.uk>

understanding/20/labels-or-instructions.html

@@ -92,7 +92,7 @@ <h2>Examples of Labels or Instructions</h2>
             correct abbreviation.
          </li>
 
-         <li>A field for entering a date contains initial text which indicates the correct format
+         <li>A field for entering a date has text instructions to indicate the correct format
             for the date.
          </li>