Document RPC method aliasing

Summary:
Suggested by Sjors Provoost <sjors@sprovoost.nl> in
https://github.com/bitcoin/bitcoin/pull/11536#issuecomment-372820660

This is a backport of Core PR12700

Test Plan:

make

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4169