Clarify closing identifier indentation behaviour of Heredoc (#5763)

Author: AlwaysVictorious

language/types/string.xml

@@ -216,8 +216,9 @@ echo 'Variables do not $expand $either', PHP_EOL;
    </simpara>
 
    <simpara>
-    The closing identifier may be indented by space or tab, in which case
-    the indentation will be stripped from all lines in the doc string.
+    The closing identifier may be indented with spaces or tabs. Whitespace
+    equal to the closing identifier's indentation is then removed from the
+    beginning of every line in the heredoc string.
     Prior to PHP 7.3.0, the closing identifier <emphasis>must</emphasis>
     begin in the first column of the line.
    </simpara>
@@ -233,20 +234,20 @@ echo 'Variables do not $expand $either', PHP_EOL;
     <programlisting role="php">
 <![CDATA[
 <?php
-// no indentation
+// closing identifier not indented
 echo <<<END
       a
      b
     c
 \n
 END;
 
-// 4 spaces of indentation
+// closing identifier indented with 3 spaces; 3 spaces stripped from each line
 echo <<<END
       a
      b
     c
-    END;
+   END;
 ]]>
     </programlisting>
     &example.outputs.73;
@@ -256,9 +257,9 @@ echo <<<END
      b
     c
 
-  a
- b
-c
+   a
+  b
+ c
 ]]>
     </screen>
    </example>