unify the devcontainer JSON and feature reference regarding mounts

The **mounts** section in https://containers.dev/implementors/json_reference/
states each value is a (**string**) matching the Docker CLI --mount flag syntax,
while https://containers.dev/implementors/features/
describes each value as an (**object**) with the same Docker CLI options.

Kindly update the documentation to unify and clarify that both string and object formats are supported.

Thank you!