docs: fix grammar for dynamic routes catch-all explanation and give better examples (vercel#48181)

### Why?

Making the explanation more obvious because it took quite a while for me
to figure out the difference between catch-all and optional catch-all,
and I think others may feel the same way too.

---------

Co-authored-by: JJ Kasper <jj@jjsweb.site>

Author: Just-Moh-it

docs/routing/dynamic-routes.md

@@ -93,7 +93,8 @@ Read our docs for [Linking between pages](/docs/routing/introduction.md#linking-
 
 Dynamic routes can be extended to catch all paths by adding three dots (`...`) inside the brackets. For example:
 
-- `pages/post/[...slug].js` matches `/post/a`, but also `/post/a/b`, `/post/a/b/c` and so on.
+- `pages/post/[...slug].js` not only matches `/post/a`, but also `/post/a/b`, `/post/a/b/c`, and so on.
+- `pages/post/[...slug].js` does not match `/post`.
 
 > **Note**: You can use names other than `slug`, such as: `[...param]`