Doc about specifying domains in domains specific backends

When enabling the domain specific backends configuration, the
user needs to specify a domain, by passing as a filter in the
call or by using a domain scoped token.

Change-Id: I5b19b6d7091df474a7b68d4ffdb052efbb8e7104
Partial-Bug: 1387379
Merge "Doc about specifying domains in domains specific backends"