Add comments about edge cases in the implementation of the functions

src/dw0c.rs

@@ -3,6 +3,12 @@ use crate::{
     rational::{rational_7_over_7, rational_8_over_7},
 };
 
+// This is an implementation of the approximation of the principal
+// branch of the Lambert W function
+// with 50 bits of accuracy from Fukushima's paper.
+// It returns f64::NAN if the input is negative or NAN,
+// and f64::INFINITY if the input is positive infinity.
+
 /// zc = z + 1/e
 pub fn dw0c(zc: f64) -> f64 {
     if zc < 0.0 || zc.is_nan() {

src/dwm1c.rs

@@ -4,6 +4,12 @@ use crate::{
     rational::rational_7_over_7,
 };
 
+// This is an implementation of the approximation of the secondary
+// branch of the Lambert W function
+// with 50 bits of accuracy from Fukushima's paper.
+// It returns f64::NAN if the input is negative, NAN, or larger than or equal to 0.
+// It returns f64::INFINITY if the input is positive infinity.
+
 /// zc = z + 1/e
 pub fn dwm1c(z: f64, zc: f64) -> f64 {
     if zc < 0.0 {

src/sw0.rs

@@ -4,6 +4,12 @@ use crate::{
     rational::{rational_3_over_3, rational_4_over_3},
 };
 
+// This is an implementation of the approximation of the principal
+// branch of the Lambert W function
+// with 24 bits of accuracy from Fukushima's paper.
+// It returns f64::NAN if the input is negative or NAN,
+// and f64::INFINITY if the input is positive infinity.
+
 pub fn sw0(z: f64) -> f64 {
     if z < NEG_INV_E || z.is_nan() {
         f64::NAN

src/sw0f.rs

@@ -6,6 +6,12 @@ use crate::{
 
 const NEG_INV_E: f32 = Z0 as f32;
 
+// This is an implementation of the approximation of the principal
+// branch of the Lambert W function
+// with 25 bits of accuracy from Fukushima's paper.
+// It returns f32::NAN if the input is negative or NAN,
+// and f32::INFINITY if the input is positive infinity.
+
 pub fn sw0f(z: f32) -> f32 {
     if z < NEG_INV_E || z.is_nan() {
         f32::NAN

src/swm1.rs

@@ -4,6 +4,12 @@ use crate::{
     rational::rational_3_over_3,
 };
 
+// This is an implementation of the approximation of the secondary
+// branch of the Lambert W function
+// with 24 bits of accuracy from Fukushima's paper.
+// It returns f64::NAN if the input is negative, NAN, or larger than or equal to 0.
+// It returns f64::INFINITY if the input is positive infinity.
+
 pub fn swm1(z: f64) -> f64 {
     if z < NEG_INV_E {
         f64::NAN

src/swm1f.rs

@@ -7,6 +7,12 @@ use crate::{
 const INV_SQRT_E: f32 = X0 as f32;
 const NEG_INV_E: f32 = Z0 as f32;
 
+// This is an implementation of the approximation of the secondary
+// branch of the Lambert W function
+// with 24 bits of accuracy from Fukushima's paper.
+// It returns f32::NAN if the input is negative, NAN, or larger than or equal to 0.
+// It returns f32::INFINITY if the input is positive infinity.
+
 pub fn swm1f(z: f32) -> f32 {
     if z < NEG_INV_E {
         f32::NAN