patch earthdistance.sgml (add geo_distance function description)

$ diff -urN earthdistance.sgml.orig earthdistance.sgml
--- earthdistance.sgml.orig     2019-01-29 00:59:46.066302968 +0900
+++ earthdistance.sgml  2019-01-29 01:07:56.814501839 +0900
@@ -120,6 +120,13 @@
       </entry>
      </row>
      <row>
+      <entry><function>geo_distance(point, point)</function><indexterm><primary>geo_distance</primary></indexterm></entry>
+      <entry><type>float8</type></entry>
+      <entry>Returns distance between the points in miles on earth's surface. A
+       point is composed with longitude(x) and latitude(y).
+      </entry>
+     </row>
+     <row>
       <entry><function>earth_box(earth, float8)</function><indexterm><primary>earth_box</primary></indexterm></entry>
       <entry><type>cube</type></entry>
       <entry>Returns a box suitable for an indexed search using the cube