Document coercers don't check result value

Closes RT#132980: https://rt.perl.org/Ticket/Display.html?id=132980

Author: zoffixznet

doc/Type/Signature.pod6

@@ -452,6 +452,11 @@ be omitted.
     foo "2016-12-01";
     # OUTPUT: «Date␤2016-12-01␤»
 
+The coercion is performed by calling the method with the name of the type
+to coerce to, if it exists (e.g. C<Foo(Bar)> coercer, would call method C<Foo>).
+The method is assumed to return the correct type—no additional checks on
+the result are currently performed.
+
 =head2 X<Slurpy (A.K.A. Variadic) Parameters|parameter,*@;parameter,*%,slurpy argument (Signature)>
 
 A function is X<variadic> if it can take a varying number of arguments; that is,