Mention that default precision is 64b in docs / add warning for 32b

[alexander-held]

Summary

The set_backend documentation currently mentions that the default precision is backend-dependent. That is technically true (the settings are assigned on a per-backend basis), but the default is 64b in all cases and as such the API documentation could highlight that. This came up via https://github.com/scikit-hep/pyhf/discussions/1921, where @kratsg suggested in addition to add a warning regarding the use of 32b: it is technically supported, but fits were seen to be unstable with it.

Documentation Page Link

https://pyhf.readthedocs.io/en/v0.6.3/_generated/pyhf.set_backend.html

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