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/admin/user.go

@@ -175,7 +175,7 @@ func EditUser(ctx *context.APIContext) {
 	// parameters:
 	// - name: username
 	//   in: path
-	//   description: username of user to edit
+	//   description: username of the user whose data is to be edited
 	//   type: string
 	//   required: true
 	// - name: body
@@ -272,7 +272,7 @@ func DeleteUser(ctx *context.APIContext) {
 	// parameters:
 	// - name: username
 	//   in: path
-	//   description: username of user to delete
+	//   description: username of the user to delete
 	//   type: string
 	//   required: true
 	// - name: purge
@@ -328,7 +328,7 @@ func CreatePublicKey(ctx *context.APIContext) {
 	// parameters:
 	// - name: username
 	//   in: path
-	//   description: username of the user
+	//   description: username of the user who is to receive a public key
 	//   type: string
 	//   required: true
 	// - name: key
@@ -358,7 +358,7 @@ func DeleteUserPublicKey(ctx *context.APIContext) {
 	// parameters:
 	// - name: username
 	//   in: path
-	//   description: username of user
+	//   description: username of the user whose public key is to be deleted
 	//   type: string
 	//   required: true
 	// - name: id
@@ -405,7 +405,7 @@ func SearchUsers(ctx *context.APIContext) {
 	//   format: int64
 	// - name: login_name
 	//   in: query
-	//   description: user's login name to search for
+	//   description: identifier of the user, provided by the external authenticator
 	//   type: string
 	// - name: page
 	//   in: query
@@ -456,7 +456,7 @@ func RenameUser(ctx *context.APIContext) {
 	// parameters:
 	// - name: username
 	//   in: path
-	//   description: existing username of user
+	//   description: current username of the user
 	//   type: string
 	//   required: true
 	// - name: body