Add explanation to which method we use to calculate coth. (#1206)

It was pointed out that there are multiple methods of calculating coth()
and it would be useful for a user to know which one we have implemented
:)

Author: gem-neo4j

modules/ROOT/pages/functions/mathematical-trigonometric.adoc

@@ -345,6 +345,7 @@ The cotangent of `0.5` is returned.
 
 | `coth(null)` returns `null`.
 | `coth(0)` returns `NaN`.
+| The calculation of the hyperbolic cotangent, `coth(x)`, is done using the `cosh(x) / sinh(x)` method.
 
 |===