device: rust: expand documentation for DeviceContext

Expand the documentation around DeviceContext states and types, in order
to provide detailed information about their purpose and relationship
with each other.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Link: https://lore.kernel.org/r/20250722150110.23565-2-dakr@kernel.org


[ Fix two minor typos. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>