doc: update readme and function descriptions.

Author: ghosind

README.md

@@ -18,7 +18,7 @@ go get -u github.com/ghosind/go-assert
 
 ## Getting Started
 
-This library provided assertion functions to verify the equality of values:
+This library provided assertion functions to verify the equality of values, or assert for nil:
 
 ```go
 func TestExample(t *testing.T) {
@@ -27,6 +27,12 @@ func TestExample(t *testing.T) {
 
   // assert inequality
   assert.NotDeepEqual(t, actual, expect)
+
+  // assert for nil
+  assert.Nil(t, object)
+
+  // assert for not nil
+  assert.NotNil(t, object)
 }
 ```
 

assertion.go

@@ -9,10 +9,6 @@ type Assertion struct {
 	t *testing.T
 }
 
-// #########################
-// ## Assertion Functions ##
-// #########################
-
 // New returns an assertion instance for verifying invariants.
 func New(t *testing.T) *Assertion {
 	a := new(Assertion)
@@ -25,6 +21,10 @@ func New(t *testing.T) *Assertion {
 	return a
 }
 
+// #########################
+// ## Assertion Functions ##
+// #########################
+
 // DeepEqual tests deeply equality between actual and expect parameters.
 func (a *Assertion) DeepEqual(actual, expect any, message ...string) error {
 	return DeepEqual(a.t, actual, expect, message...)
@@ -35,12 +35,16 @@ func (a *Assertion) NotDeepEqual(actual, expect any, message ...string) error {
 	return NotDeepEqual(a.t, actual, expect, message...)
 }
 
-// Nil tests a value is nil or not, and it'll failed when the value is not nil.
+// Nil tests whether a value is nil or not, and it'll fail when the value is not nil. It will
+// always return false if the value is a bool, an integer, a floating number, a complex, or a
+// string.
 func (a *Assertion) Nil(val any, message ...string) error {
 	return Nil(a.t, val, message...)
 }
 
-// NotNil tests a value is nil or not, and it'll failed when the value is nil.
+// NotNil tests whether a value is nil or not, and it'll fail when the value is nil. It will
+// always return true if the value is a bool, an integer, a floating number, a complex, or a
+// string.
 func (a *Assertion) NotNil(val any, message ...string) error {
 	return NotNil(a.t, val, message...)
 }

equal.go

@@ -32,7 +32,9 @@ func NotDeepEqual(t *testing.T, actual, expect any, message ...string) error {
 	return err
 }
 
-// Nil tests a value is nil or not, and it'll failed when the value is not nil.
+// Nil tests whether a value is nil or not, and it'll fail when the value is not nil. It will
+// always return false if the value is a bool, an integer, a floating number, a complex, or a
+// string.
 func Nil(t *testing.T, val any, message ...string) error {
 	if isNil(val) {
 		return nil
@@ -45,7 +47,9 @@ func Nil(t *testing.T, val any, message ...string) error {
 	return err
 }
 
-// NotNil tests a value is nil or not, and it'll failed when the value is nil.
+// NotNil tests whether a value is nil or not, and it'll fail when the value is nil. It will
+// always return true if the value is a bool, an integer, a floating number, a complex, or a
+// string.
 func NotNil(t *testing.T, val any, message ...string) error {
 	if !isNil(val) {
 		return nil