docs(lettables): add section about known issues

In particular known issues with TS 2.3 and under not inferring types in a developer-friendly way.

closes #2869

doc/lettable-operators.md

@@ -81,3 +81,25 @@ interval(1000).pipe(
 .subscribe(x => console.log(x));
 // [0, 12, 24]
 ```
+
+## Known Issues
+
+In TypeScript 2.3 and lower, typings will need to be added to functions passed to operators, as types cannot be inferred prior to TypeScript 2.4. In TypeScript 2.4, types will infer via composition properly.
+
+**TS 2.3 and under**
+
+```ts
+range(0, 10).pipe(
+  map((n: number) => n + '!'),
+  map((s: string) => 'Hello, ' + s),
+).subscribe(x => console.log(x))
+```
+
+**TS 2.4 and up**
+
+```ts
+range(0, 10).pipe(
+  map(n => n + '!'),
+  map(s => 'Hello, ' + s),
+).subscribe(x => console.log(x))
+```