Update Timeseries API documentation to clarify "unit" query parameter

[rma-rripken]

Despite the current documentation "Other" is not an actual valid input for the unit query parameter.

The documentation should be clarified.

The desired behavior was for users to be able to specify a unit system and they would get values in the default unit for the parameter from that system
OR
the user could specify the exact unit they wanted "mm" or "m" and they would get values in the specified unit.

Maybe A new unit-system query parameter that accepts only EN/SI would be a clearer way to accomplish this?

[rma-rripken]

#622

[MikeNeilson]

Looking at it, I think just removing the text "Other" would be sufficient. But maybe update the string to:

Any measurement unit appropriate to the data requested. Units supported by the system are available at /units ... and then it's probably time to update the units endpoint.