Document Code.is-implementation-detail method

Author: Altai-man

doc/Type/Code.pod6

@@ -187,6 +187,15 @@ Returns the line number in which the code object was declared.
 
     say &infix:<+>.line;
 
+=head2 method is-implementation-detail
+
+    method is-implementation-detail(--> False)
+
+Note: this trait is available in Rakudo compiler starting from 2020.05 release.
+
+Returns C<True> if code object was marked with C<is implementation-detail> trait,
+C<False> otherwise.
+
 =end pod
 
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