Document and clean up subscriptions client limit behaviour

[tbarsballe]

Related Issue(s):

Closes #1069 Closes #1070

Proposed Changes:

For inclusion in changelog (if applicable):

1. Removed the unused limit parameter of SubscriptionsClient.get_results_csv

Not intended for changelog:

1. Added documentation of the limit=0 behaviour to other SubscriptionsClient methods where this is still used(list_subscriptions, get_results)

PR Checklist:

• [x] This PR is as small and focused as possible
• [x] If this PR includes proposed changes for inclusion in the changelog, the title of this PR summarizes those changes and is ready for inclusion in the Changelog.
• [x] I have updated docstrings for function changes and docs in the 'docs' folder for user interface / behavior changes
• [x] This PR does not break any examples or I have updated them

[asonnenschein]

Confirming after reviewing this PR and Subscription API's /results endpoint that limit is undocumented and unused, and I agree that this should be removed.