Java: Update help and description of java/suspicious-date-format.

Author: chrisgavin

java/ql/src/Likely Bugs/Likely Typos/SuspiciousDateFormat.qhelp

@@ -6,9 +6,12 @@
 
 <overview>
 <p>
-Some <code>SimpleDateFormat</code> patterns might not work correctly at the end of the calendar
-year, due to use of the <code>Y</code> placeholder (representing the ISO 8601 week year), rather
-than <code>y</code> representing the actual year.
+  The Java <code>SimpleDateFormat</code> class provides many placeholders so that you can define
+  precisely the date format required. However, this also makes it easy to define a pattern that
+  doesn't behave exactly as you intended. The most common mistake is to use the <code>Y</code>
+  placeholder (which represents the ISO 8601 week year), rather than <code>y</code> (which
+  represents the actual year). In this case, the date reported will appear correct until the end
+  of the year, when the "week year" may differ from the actual year.
 </p>
 </overview>
 
@@ -24,6 +27,9 @@ than <code>y</code> representing the actual year.
   On the 30th of December 2019, this code will output "2020-12-30", rather than the intended "2019-12-30".
 </p>
 <sample src="SuspiciousDateFormat.java" />
+<p>
+  The correct pattern in this case would be <code>yyyy-MM-dd</code> instead of <code>YYYY-MM-dd</code>.
+</p>
 </example>
 
 <references>

java/ql/src/Likely Bugs/Likely Typos/SuspiciousDateFormat.ql

@@ -1,6 +1,6 @@
 /**
  * @name Suspicious date format
- * @description Some date format patterns don't work as they might seem.
+ * @description Using a data format that includes both 'M' and 'Y' is likely to give unexpected results.
  * @kind problem
  * @problem.severity warning
  * @precision high