add _auth tests and update _auth docs

[wraithgar]

Related to #3573 and comments in #2453.

[wraithgar]

I'm going to make another commit to this PR updating the description of _auth to outline this

[npm-robot]

no statistically significant performance changes detected

timing results

app-large	clean	lock-only	cache-only	cache-only peer-deps	modules-only	no-lock	no-cache	no-modules	no-clean	no-clean audit
npm@8	60.681 ±2.87	34.139 ±0.38	25.074 ±7.50	22.629 ±0.77	3.378 ±0.05	3.373 ±0.02	2.771 ±0.10	12.693 ±0.13	2.737 ±0.04	3.840 ±0.04
#4782	55.649 ±4.53	33.610 ±0.19	31.308 ±16.38	22.165 ±0.94	3.271 ±0.01	3.341 ±0.02	2.661 ±0.09	12.856 ±0.26	2.638 ±0.02	4.062 ±0.03

app-medium	clean	lock-only	cache-only	cache-only peer-deps	modules-only	no-lock	no-cache	no-modules	no-clean	no-clean audit
npm@8	42.094 ±0.99	25.721 ±0.15	14.492 ±0.02	15.846 ±0.05	3.018 ±0.01	3.084 ±0.08	2.867 ±0.19	9.698 ±0.31	2.599 ±0.04	3.475 ±0.12
#4782	41.761 ±0.91	25.959 ±0.47	14.662 ±0.07	15.938 ±0.49	3.100 ±0.03	2.978 ±0.01	2.737 ±0.07	9.497 ±0.22	2.508 ±0.02	3.438 ±0.02