Improve expmod doc

- Clarify what it does for non-mathy types
- Document candidate that can take non-Int types

doc/Type/Int.pod6

@@ -48,10 +48,13 @@ Example:
 
 Defined as:
 
+    multi sub    expmod(      $x,     $y,     $mod) returns Int:D
     multi sub    expmod(Int:D $x, Int $y, Int $mod) returns Int:D
     multi method expmod(Int:D:    Int $y, Int $mod) returns Int:D
 
-Returns the given C<Int> raised to the C<$y> power within modulus C<$mod>.
+Returns the given C<Int> raised to the C<$y> power within modulus C<$mod>,
+that is gives the result of C<($x ** $y) mod $mod>. The subroutine form
+can accept non-C<Int> arguments, which will be coerced to C<Int>.
 
     say expmod(4, 2, 5);    # 1
     say 7.expmod(2, 5);     # 4