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/repo/issue_subscription.go

@@ -43,7 +43,7 @@ func AddIssueSubscription(ctx *context.APIContext) {
 	//   required: true
 	// - name: user
 	//   in: path
-	//   description: user to subscribe
+	//   description: username of the user to subscribe the issue to
 	//   type: string
 	//   required: true
 	// responses:
@@ -87,7 +87,7 @@ func DelIssueSubscription(ctx *context.APIContext) {
 	//   required: true
 	// - name: user
 	//   in: path
-	//   description: user witch unsubscribe
+	//   description: username of the user to unsubscribe from an issue
 	//   type: string
 	//   required: true
 	// responses: