Fix bug in std dirs drop; improve documentation (#9449) #15

Job	Run time
plugins (windows-latest, stable)	9m 39s
plugins (macos-latest, stable)	5m 56s
plugins (ubuntu-20.04, stable)	4m 22s
nu-tests (windows-latest, default, stable)	15m 2s
nu-tests (macos-latest, default, stable)	23m 1s
nu-tests (ubuntu-20.04, default, stable)	12m 7s
nu-tests (ubuntu-20.04, dataframe, stable)	14m 12s
nu-tests (ubuntu-20.04, extra, stable)	11m 1s
nu-fmt-clippy (windows-latest, default, stable)	8m 11s
nu-fmt-clippy (windows-latest, extra, stable)	10m 32s
nu-fmt-clippy (macos-latest, default, stable)	6m 8s
nu-fmt-clippy (macos-latest, extra, stable)	6m 55s
nu-fmt-clippy (ubuntu-20.04, default, stable)	3m 14s
nu-fmt-clippy (ubuntu-20.04, dataframe, stable)	3m 31s
nu-fmt-clippy (ubuntu-20.04, extra, stable)	3m 39s
nu-coverage	0s
std-lib-and-python-virtualenv (ubuntu-20.04, stable, py)	6m 21s
std-lib-and-python-virtualenv (macos-latest, stable, py)	14m 45s
std-lib-and-python-virtualenv (windows-latest, stable, py)	13m 4s
	2h 51m 40s

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