Added more sample output for /transactions response

Summary: Done with 70% of the transaction types, will put this ticket back to backlog

Test Plan: N/A

Reviewers: alchan

Differential Revision: https://phabricator.dev.oanda.com/D2074
Updated doc for ETag changes

Summary: Document changes for ETag

Test Plan: N/A

Reviewers: alchan, cyip

Differential Revision: http://phabricator.dev.oanda.com:7600/D1755