docs(no-explicit-generics): add known union problem

Author: JasonWeinzierl

docs/rules/no-explicit-generics.md

@@ -19,3 +19,7 @@ Examples of **correct** code for this rule:
 import { BehaviorSubject } from "rxjs";
 const subject = new BehaviorSubject(42);
 ```
+
+## Known problems
+
+- ([#77](https://github.com/JasonWeinzierl/eslint-plugin-rxjs-x/issues/77)) Type unions cause false positives e.g. `new BehaviorSubject<number | null>(null)` will be incorrectly caught by this rule.