Docs improvements

[mtvch]

Hi!
I have been reading the sources recently and found a few typos along with a link to a wrong place in brod_group_subscriber_v2.

[zmstone]

I believe it should be brod_topic_subscriber:start_link/7 ?

[mtvch]

I'm sure that consumer_config option accepts config that is passed to brod_consumer:start_link/5, but here in brod_group_consumer_v2 a link leads to brod_topic_subscriber where other irrelevant options are described.

Please, compare brod_consumer config:
https://hexdocs.pm/brod/brod_consumer.html#start_link/5
WIth brod_topic_subscriber config:
https://hexdocs.pm/brod/brod_topic_subscriber.html#start_link/1

[zmstone]

consumer_group is used here:

brod/src/brod_group_subscriber_v2.erl

Lines 510 to 521 in 5172dbe

	start_worker(Client, Topic, MessageType, Partition, ConsumerConfig,
	StartOptions) ->
	{ok, Pid} = brod_topic_subscriber:start_link( Client
	, Topic
	, [Partition]
	, ConsumerConfig
	, MessageType
	, brod_group_subscriber_worker
	, StartOptions
	),
	{ok, Pid}.

[mtvch]

What I am trying to say is that the link in the description of consumer_config option does not lead to a description of said options, which confused me a bit.

consumer_config options are eventually passed to brod_consumer:start_link/5 and I believe there should be a link to this function, because they are described in it's documentation.

But currently there is a link to brod_topic_subscriber:start_link/1 where docs describe only config of brod_topic_subscriber. consumer_config is one of the options in brod_topic_subscriber's config, but even there a link leads to brod_topic_subscriber itself. I'm going to fix this too in this PR.

In addition, in other places like in brod_group_subscriber description of ConsumerConfig option has a link to brod_consumer docs as I'm suggesting.