ext/intl: Add negative $offset for grapheme_strr functions (#4864)

* Add negative $offset for grapheme_strr functions

Co-authored-by: Jordi Kroon <jordikroon@php.net>

Author: youkidearitai

reference/intl/grapheme/grapheme-strripos.xml

@@ -49,6 +49,9 @@
        The position returned is still relative to the beginning of <parameter>haystack</parameter>
        regardless of the value of <parameter>offset</parameter>.
       </para>
+      <simpara>
+       The optional offset parameter allows you to specify where in <parameter>haystack</parameter> to start searching as an <parameter>offset</parameter> in grapheme units (not bytes or characters). If the <parameter>offset</parameter> is negative, it is treated relative to the end of the string. The position returned is still relative to the beginning of <parameter>haystack</parameter> regardless of the value of offset. The search is performed right to left, searching for the first occurrence of <parameter>needle</parameter> from the selected grapheme cluster.
+      </simpara>
      </listitem>
     </varlistentry>
     <varlistentry>

reference/intl/grapheme/grapheme-strrpos.xml

@@ -49,6 +49,9 @@
        The position returned is still relative to the beginning of <parameter>haystack</parameter>
        regardless of the value of <parameter>offset</parameter>.
       </para>
+      <simpara>
+       The optional offset parameter allows you to specify where in <parameter>haystack</parameter> to start searching as an <parameter>offset</parameter> in grapheme units (not bytes or characters). If the <parameter>offset</parameter> is negative, it is treated relative to the end of the string. The position returned is still relative to the beginning of <parameter>haystack</parameter> regardless of the value of offset. The search is performed right to left, searching for the first occurrence of <parameter>needle</parameter> from the selected grapheme cluster.
+      </simpara>
      </listitem>
     </varlistentry>
     <varlistentry>