Wrong or misleading function names in protocol

[jarlah]

This

https://github.com/testcontainers/testcontainers-elixir/blob/main/lib/container/protocols/container_builder.ex#L9

is saying something in the lines of "return ok if the container is starting", but its really a "call this function if the container has started".

Consider better naming of the function and add docs

[jarlah]

also fix the kafka container documentation for this method