docs: add note about possible TS inference problems with FilterQuery

Closes #208

docs/entity-manager.md

@@ -99,7 +99,7 @@ for (const author of authors) {
 }
 ```
 
-### Conditions Object
+### Conditions Object (FilterQuery<T>)
 
 Querying entities via conditions object (`where` in `em.find(Entity, where: FilterQuery<T>)`) 
 supports many different ways:
@@ -134,6 +134,29 @@ As you can see in the fifth example, one can also use operators like `$and`, `$o
 `$gt`, `$lte`, `$lt`, `$in`, `$nin`, `$eq`, `$ne`. More about that can be found in 
 [Query Conditions](query-conditions.md) section. 
 
+#### Mitigating `Type instantiation is excessively deep and possibly infinite.ts(2589)` error
+
+Sometimes you might be facing TypeScript errors caused by too complex query for it to 
+properly infer all types. Usually it can be solved by providing the type argument 
+explicitly.
+
+You can also opt in to use repository instead, as there the type inference should not be
+problematic. 
+
+> As a last resort, you can always type cast the query to `any`.
+
+```typescript
+const books = await orm.em.find<Book>(Book, { ... your complex query ... });
+// or
+const books = await orm.em.getRepository(Book).find({ ... your complex query ... });
+// or
+const books = await orm.em.find<any>(Book, { ... your complex query ... }) as Book[];
+```
+
+Another problem you might be facing is `RangeError: Maximum call stack size exceeded` error 
+thrown during TypeScript compilation (usually from file `node_modules/typescript/lib/typescript.js`).
+The solution to this is the same, just provide the type argument explicitly.
+
 ### Searching by referenced entity fields
 
 You can also search by referenced entity properties. Simply pass nested where condition like 

tests/EntityManager.mysql.test.ts

@@ -1376,8 +1376,12 @@ describe('EntityManagerMySql', () => {
     author.books.add(book1, book2, book3);
     await orm.em.persistAndFlush(author);
     await expect(orm.em.count(Book2, [book1.uuid, book2.uuid, book3.uuid])).resolves.toBe(3);
-    // this test was causing TS recursion errors, see https://github.com/mikro-orm/mikro-orm/issues/124
-    // await expect(orm.em.count(Book2, [book1, book2, book3])).resolves.toBe(3);
+    // this test was causing TS recursion errors without the type argument
+    // see https://github.com/mikro-orm/mikro-orm/issues/124 and https://github.com/mikro-orm/mikro-orm/issues/208
+    await expect(orm.em.count<Book2>(Book2, [book1, book2, book3])).resolves.toBe(3);
+    await expect(orm.em.count<any>(Book2, [book1, book2, book3])).resolves.toBe(3);
+    const a = await orm.em.find<any>(Book2, [book1, book2, book3]) as Book2[];
+    await expect(orm.em.getRepository(Book2).count([book1, book2, book3])).resolves.toBe(3);
   });
 
   test('find by joined property', async () => {