Clarify parsing just the year after two notes were created for this

Author: derickr

reference/datetime/formats.xml

@@ -556,9 +556,15 @@ object(DateTimeImmutable)#1 (3) {
 
   <note>
    <para>
-    The "Year (and just the year)" format only works if a time string
-    has already been found -- otherwise this format is recognised as
-    <literal>HH</literal> <literal>MM</literal>.
+    The "Year (and just the year)" format only reliably works if a time string
+    has already been found. Otherwise, if the four digit year matches
+    <literal>HH</literal> <literal>MM</literal> then these two date elements
+    are set instead.
+   </para>
+   <para>
+    To consistently parse just a year, use
+    <function>DateTimeImmutable::createFromFormat</function> with the
+    <literal>Y</literal> specifier.
    </para>
   </note>