jax.devices returns only the devices from the default backend

[gnecula]

The documentation of jax.devices says that it returns all devices. This is not true in multi-backend setups, it returns only the devices from the default backend. This was confusing to me initially, and I also encountered this confusion in issue #2785

The simplest change would be to the documentation: if no backend is specified then only the devices on the default backend are returned (along with an explanation of what is the default backend).

A better change may be though to say that it returns all devices, in the order of backend priority, such that devices()[0] is the same as now. This may break some code though.

[gnecula]

I searched in Google3 and could not find any code that uses the backend parameter for jax.devices other than tests.