Add note to structlog docs

[will-ockmore]

Make it clear that the cache_logger_on_first_use parameter should not be set during tests.

[etianen]

I wonder, can we disable the cache during log capture by some structlog method call?

[will-ockmore]

We could (with the configure call in start_capturing). This wouldn't affect any previously cached loggers; there's no way to reconfigure them once initialised without access to the instance.

See the second point on https://www.structlog.org/en/stable/performance.html for a more detailed explanation of the mechanism.

It would be down to the user to ensure no loggers are cached at any stage; even if we set a global hook they could always call configure again with cache_logger_on_first_use and then initialise a logger somewhere

[etianen]

Ah, and even the testing docs for `structlog' say the same thing: turn off this in testing!