Added documentation for sqrt

Author: Jan-Olof Hendig

doc/Type/Complex.pod6

@@ -158,6 +158,19 @@ sign of the imaginary part negated).
 
     say (1-4i).conj;                # OUTPUT: «1+4i␤»
 
+=head2 method sqrt
+
+Defined as:
+
+    method sqrt(Complex:D: --> Complex:D)
+
+Returns the complex square root of the invocant, i.e. the root
+where the real part is ≥ 0 and the imaginary part has the same
+sign as the imaginary part of the invocant.
+
+    say (3-4i).sqrt;                # OUTPUT: «2-1i␤»
+    say (-3+4i).sqrt;               # OUTPUT: «1+2i␤»
+
 =head2 method gist
 
 Defined as: