Clarify that using block_identifier with .call() comes with its caveats

[miohtama]

To manage the expectations of the client library users.

What was wrong?

Programmers were using call with a block number, but did not know it might not work as it.

How was it fixed?

Document block_identifier behavior with various Ethereum nodes.

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[miohtama]

Looks like the docs fails in Circle CI, but when I click "Details" it asks me for a login. Are the CI reports publicly available?

[kclowes]

Thanks for the clarification @miohtama! You should definitely be able to see the CI from a branch you wrote but you may need to log in to CircleCI with your Github creds. But to fix the docs build, you just need to unindent the paragraph you wrote. The geth failure is a flaky failure.

[kclowes]

Thanks!