services/horizon: Add Protocol 14 examples to docstrings

[Shaptic]

Per @ire-and-curses, we (I) need to update the doc-strings to include examples for the new operations introduced in Protocol 14, much like the unit-testable examples for existing operations (e.g. Payment). This includes:

• create claimable balance
• claim claimable balance
• begin/end sponsoring future reserves
• revoke sponsorship

It should be straightforward to adapt them from the official SDK examples (e.g. these for the latter two ops) in the new docs.