Add a reference to pow to the description of exp.

This might be especially usefull since godot script doesn't support ** or ^ as operators, so beginners might search for the exponential function, when what they really need is the pow function.

This is exactly what happened to me and since I couldn't find helpfull information in the documentation I had to look it up online, where I found the answer on a helpfull [reddit thread](https://www.reddit.com/r/godot/comments/3mvwz0/how_do_i_do_exponents_in_godot/).

@akien-mga told me how to reference methods here:
godotengine#30909

(cherry picked from commit a2c81a7455)

modules/gdscript/doc_classes/@GDScript.xml

@@ -329,6 +329,7 @@
 			<description>
 				The natural exponential function. It raises the mathematical constant [b]e[/b] to the power of [code]s[/code] and returns it.
 				[b]e[/b] has an approximate value of 2.71828.
+				For exponents to other bases use the method [method pow].
 				[codeblock]
 				a = exp(2) # approximately 7.39
 				[/codeblock]