placement: Document `in:` prefix for ?member_of=
placement: Document `in:` prefix for ?member_of=

The documentation for the member_of query parameter for GET
/resource_providers [1] claims that it accepts a "comma-separated list
of strings representing aggregate uuids".  But it turns out the code [2]
is expecting a single UUID unless the prefix 'in:' is specified.

This change set updates the documentation accordingly.

[1] https://developer.openstack.org/api-ref/placement/#resource-providers
[2] https://github.com/openstack/nova/blob/57728836f25e9201ddec1e7790b552cfa840d572/nova/api/openstack/placement/handlers/resource_provider.py#L229-L233

Change-Id: I2ec46d767ded5be02cfc73136f2d217f1347662a
Closes-Bug: #1733030

phenoxim/nova,mikalstill/nova,mahak/nova,rahulunair/nova,gooddata/openstack-nova,klmitch/nova,mahak/nova,rahulunair/nova,gooddata/openstack-nova,openstack/nova,rahulunair/nova,mikalstill/nova,gooddata/openstack-nova,klmitch/nova,openstack/nova,gooddata/openstack-nova,mahak/nova,phenoxim/nova,klmitch/nova,openstack/nova,klmitch/nova,mikalstill/nova