Document api loc/facts/(add|get|rem) with optional "id" parameter

The https://github.com/Comcast/rulio/blob/master/doc/api/rules-core-http-api.md document should be updated to reflect that /loc/facts/(add | get | rem) can be passed an optional "id" parameter.

Open doc/api/rules-core-http-api.md and review the documentation for /loc/facts/(add | get | rem). Update the endpoint descriptions to mention the optional "id" parameter; done means the document accurately reflects that parameter for all three operations.

Written by the indexing model from the issue text.