Clarify stdJson key syntax (#832)

The `stdJson` docs currently use a generic `<json_path>` placeholder,
but the API actually expects the same selector syntax as
`vm.parseJson*`. The existing tests already show the real usage with
`.a` for a field and `$` for the root object in `test/StdJson.t.sol`, so
the current comments are easy to misread.
This updates the `StdJson` examples and function docs to match the
behavior that is already implemented and tested today. It is a
comment-only change with no ABI or runtime impact, just a documentation
fix that makes the interface clearer and more accurate.

Co-authored-by: zerosnacks <95942363+zerosnacks@users.noreply.github.com>