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.go

@@ -15,9 +15,9 @@ import (
 type User struct {
 	// the user's id
 	ID int64 `json:"id"`
-	// the user's username
+	// login of the user, same as `username`
 	UserName string `json:"login"`
-	// the user's authentication sign-in name.
+	// identifier of the user, provided by the external authenticator (if configured)
 	// default: empty
 	LoginName string `json:"login_name"`
 	// The ID of the user's Authentication Source