added new help files for geometry functions

resources/function_help/convexHull

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+<h3>convexHull function</h3>
+Returns the convex hull of a geometry. It represents the minimum convex geometry that encloses all geometries within the set.
+
+<h4>Syntax</h4>
+<pre>convexHull( $geometry , $geometry)</pre>
+
+<h4>Arguments</h4>
+geometry &rarr; geometry
+geometry &rarr; geometry
+
+<h4>Example</h4>
+<pre>   &rarr; returns </pre>