expanded on No Router Instance (vercel#21248)

Added more detailed explanation of why there is no router instance, and other ways to fix the error

errors/no-router-instance.md

@@ -2,8 +2,12 @@
 
 #### Why This Error Occurred
 
-During SSR you might have tried to access a router method `push`, `replace`, `back`, which is not supported.
+During Pre-rendering (SSR or SSG) you tried to access a router method `push`, `replace`, `back`, which is not supported.
 
 #### Possible Ways to Fix It
 
-Move any calls to router methods to `componentDidMount` or add a check such as `typeof window !== 'undefined'` before calling the methods
+In a function Component you can move the code into the `useEffect` hook.
+
+In a class Component, move any calls to router methods to the `componentDidMount` lifecycle method.
+
+This way the calls to the router methods are only executed in the browser.