C API: document nix_state_create() needs a corresponding nix_state_free()

[Radvendii]

Is your feature request related to a problem? Please describe.

The C API docs tend to say how the user needs to properly dispose of the values returned. e.g. nix_c_context_create() says "Owned by caller. Free using nix_c_context_free()"

nix_state_create() has a similar property where it's owned by the caller and needs a nix_state_free() call, but this isn't documented.

Describe the solution you'd like
Document the usage of nix_state_create()/nix_state_free() in the same way as nix_c_context_create()/nix_c_context_free()

Describe alternatives you've considered
The documentation already has a link to the nix_state_free() function, which is sort of clear enough, but for instance it doesn't show up in the rust documentation when you generate c bindings. Regardless, the documentation should be consistent between different functions.

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