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

routers/api/v1/user/app.go

@@ -30,7 +30,7 @@ func ListAccessTokens(ctx *context.APIContext) {
 	// parameters:
 	// - name: username
 	//   in: path
-	//   description: username of user
+	//   description: username of to user whose access tokens are to be listed
 	//   type: string
 	//   required: true
 	// - name: page
@@ -83,7 +83,7 @@ func CreateAccessToken(ctx *context.APIContext) {
 	// parameters:
 	// - name: username
 	//   in: path
-	//   description: username of user
+	//   description: username of the user whose token is to be created
 	//   required: true
 	//   type: string
 	// - name: body
@@ -149,7 +149,7 @@ func DeleteAccessToken(ctx *context.APIContext) {
 	// parameters:
 	// - name: username
 	//   in: path
-	//   description: username of user
+	//   description: username of the user whose token is to be deleted
 	//   type: string
 	//   required: true
 	// - name: token