Update es_client_authentication description

[tobio]

Fixes #956

@Leaf-Lin is this the change you're after (you can just look at the first commit)? Otherwise, want to let me know what you'd like this to say?

[Leaf-Lin]

Thank you for the quick fix! I didn't realise it has been mentioned in so many different places. This will clear up the usage of the variable.

[tobio]

Thank you for the quick fix! I didn't realise it has been mentioned in so many different places. This will clear up the usage of the variable.

All good, the docs are generated so it's only one code change.