explain coersion and hint at Cool

Author: gfldex

doc/Language/typesystem.pod6

@@ -53,6 +53,20 @@ L<//|/language/operators#infix_//>,
  L<with/without|/language/control#with,_orwith,_without> and
 L<signatures|/type/Signature#Constraining_Defined_and_Undefined_Values>.
 
+=head3 Coersion
+
+Turning one type into another is done with coersion methods that have the same
+name then the target type. This convention is made mandatory by
+L<Signatures|/type/Signature#Coercion_Type>. The source type has to know how to
+turn itself into the target type. To allow build-in types to turn themself into
+user defined types use L<augment|/language/variables#The_augment_Declarator>.
+
+Perl 6 provides methods defined in L<Cool|/type/Cool> to convert to a target
+type before applying further operations. Most build-in types decend from
+C<Cool> and as such may provide implicit coersion that may be undesired. It is
+the responsibility of the user to care about trap-free usage of those
+methods.
+
 =head1 Type Declarators
 
 Type declarators introduce a new type into the given scope. Nested scopes can