Adds the {:error, :no_cache} case to function doc to be clear

[tpickell-papa]

We ran into this in production and the developers that wrote the code using Cachex did not know that
get could return anything other than {:ok, "value"} or {:ok, nil}
When looking at the docs for the function call I can see why.
This may be helpful for others using this lib in their code.

[feld]

It might be better to update the @spec to

 @spec get(cache, any, Keyword.t()) :: {:ok, any()} | {:error, :no_cache}

[feld]

the {:error, :no_cache} should be added to the @spec of get_and_update/4 -- just ran into this as well

[whitfin]

Given that the specs have been updated by @feld I'm going to close this; it should be communicated more clearly now. Thank you for raising this regardless!