Document how to disable steep in an easy to find location

[p-datadog]

https://github.com/soutaro/steep/pull/1034 (as referenced from https://github.com/soutaro/steep/issues/1232) implemented a way to disable steep for a block of code. However, this functionality for some reason was not/is not documented.

Since this is a basic operation when one uses steep, it should be either mentioned in the top-level readme or it could be in a guide but it should still be straightforward to find.