docs(indexer): more clarity for ChainIndexer configuration setup

[rvagg]

Feedback already suggesting that our lack of automagic config switching is leading to some confusion. I'd still rather avoid automatically turning on ChainIndexer in these cases so we'll try with more clear error messages and see if we continue to get reports.

[github-actions]

Please update the PR title to match https://github.com/filecoin-project/lotus/blob/master/CONTRIBUTING.md#pr-title-conventions

PR title now matches the required format.

[virajbhartiya]

I am not sure but will it be a good idea to add a commented-out config for chainidnexer the way EnableEthRPC = true is set to true but commented in the config file by default so that it is easier for a new user.

[rjan90]

Needs a make gen/make docsgen it seems, but apart from that, lgtm.

[rvagg]

commented-out config for chainidnexer

unfortunately the default config is generated from settings automatically, and their values are meant to reflect what they are currently set to - so they are a kind of documentation for the user, demonstrating what the current settings are, so they have to change them if they want them.

[rvagg]

ahh, actually needs a docsgen-cli because of the config file output, gen and docsgen don't cut it (I ran them already!)