Improve documentation of `check_deterministic`

[runame]

In many cases users probably do not want to let check_determinisic default to True, due to the additional cost. We should document recommended workflows for this, maybe using warnings or just in the docstring.

[f-dangel]

Personally, I really like that the deterministic checks are turned on by default. This has saved many of my students from running into numerical artifacts. I agree though it should be easier for users to identify that there is an expensive check carried out every time they instantiate a linear operator, and that this can be turned off after verifying that things work (or it should only be run once, then turned off).

Some suggestions that make it easier for users to notice this:

• We could implement this as a warning
• Or, we could start an 'FAQ'/'Important' section in the README