Add explanation to which method we use to calculate coth.

Author: gem-neo4j

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

@@ -331,6 +331,8 @@ The cotangent of `0.5` is returned.
 [[functions-coth]]
 == coth()
 
+The calculation of the hyperbolic cotangent, `coth(x)`, is done using the `cosh(x) / sinh(x)` method.
+
 .Details
 |===
 | *Syntax* 3+| `coth(input)`