Add a login/login-name/username disambiguation to affected endpoint parameters and response/request models (#34901)

Issue: [link](https://github.com/go-gitea/gitea/issues/9637)

Changes introduced: I have clarified the problematic terms (`login`,
`login_name`, and `username`) in all affected endpoints.

The changes were made to relevant:
- HTTP endpoint parameters' descriptions 
- response/request models' fields

modules/structs/hook.go

@@ -71,7 +71,8 @@ type PayloadUser struct {
 	// Full name of the commit author
 	Name string `json:"name"`
 	// swagger:strfmt email
-	Email    string `json:"email"`
+	Email string `json:"email"`
+	// username of the user
 	UserName string `json:"username"`
 }