fix and add links to NaN

Author: gfldex

doc/Type/Complex.pod

@@ -86,7 +86,7 @@ Usage:
 
     COMPLEX.isNaN
 
-Returns true if the real or imaginary part is C<NaN> (not a number).
+Returns true if the real or imaginary part is L<C<NaN>|/type/Num#NaN> (not a number).
 
 =head2 method polar
 

doc/Type/Int.pod

@@ -198,7 +198,7 @@ Usage:
     INTEGER.unival
 
 Returns the number represented by the Unicode codepoint with the given integer
-number, or L<NaN> if it does not represent a number.
+number, or L<NaN|/type/Num#NaN> if it does not represent a number.
 
     say ord("¾").unival;    # 0.75
     say 190.unival;         # 0.75

doc/Type/Numeric.pod

@@ -113,7 +113,7 @@ Returns the absolute value of the number.
 Returns a square root of the number. For real numbers the positive square
 root is returned.
 
-On negative real numbers, C<sqrt> returns C<NaN> rather than a complex number,
+On negative real numbers, C<sqrt> returns L<C<NaN>|/type/Num#NaN> rather than a complex number,
 in order to not confuse people who are not familiar with complex arithmetic.
 If you want to calculate complex square roots, coerce to C<Complex> first, or
 use the C<roots> method.