* ENH: Update to add link to live QE docs

* ENH: Update asset_identifier_format default to sid

asset_identifier_format values of `symbol` and `fsym_region_id` are _only valid_ for US_EQUITIES domains.

`symbol` will cause a `TypeError: Expected unicode, got <type 'str'>` exception, while `fsym_region_id` will cause `IndexError: cannot do a non-empty take from an empty axes.`