math: add missing docstrings for math.log_gamma_sign and for the complex.complex constructor function (#19523)

vlib/math/complex/complex.v

@@ -12,6 +12,7 @@ pub mut:
 	im f64
 }
 
+// complex returns a complex struct with the given `re` and `im`
 pub fn complex(re f64, im f64) Complex {
 	return Complex{re, im}
 }
@@ -30,6 +31,7 @@ pub fn (c Complex) abs() f64 {
 	return math.hypot(c.re, c.im)
 }
 
+// mod returns the modulus value of `c`
 pub fn (c Complex) mod() f64 {
 	return c.abs()
 }

vlib/math/gamma.v

@@ -139,6 +139,7 @@ pub fn log_gamma(x f64) f64 {
 	return y
 }
 
+// log_gamma_sign returns the natural logarithm and sign (-1 or +1) of Gamma(x)
 pub fn log_gamma_sign(a f64) (f64, int) {
 	mut x := a
 	ymin := 1.461632144968362245