fix: validation (better #8456) (#8461)

* Revert "revert 484e023c0"

This reverts commit c03b70c949.

* also allow pure renote

* fix checks for pure renote

packages/backend/src/server/api/endpoints/mute/create.ts

@@ -38,7 +38,11 @@ export const paramDef = {
 	type: 'object',
 	properties: {
 		userId: { type: 'string', format: 'misskey:id' },
-		expiresAt: { type: 'integer', nullable: true },
+		expiresAt: {
+			type: 'integer',
+			nullable: true,
+			description: 'A Unix Epoch timestamp that must lie in the future. `null` means an indefinite mute.',
+		},
 	},
 	required: ['userId'],
 } as const;