[IMPROVE] Expose Index Choices In obb.fixedincome.bond_indices Docstring

[deeleeramone]

This function, from FRED, has choices that rely on other parameters, but the potential choices are not exposed to the docstring.

The index parameter should have a set of choices covering all "category" permutations that is included to the model's json_schema_extra. The parameter is validated, so it is a matter of flattening the mapping dictionary as a list.