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/user_email.go

@@ -11,6 +11,7 @@ type Email struct {
 	Verified bool   `json:"verified"`
 	Primary  bool   `json:"primary"`
 	UserID   int64  `json:"user_id"`
+	// username of the user
 	UserName string `json:"username"`
 }