Backport

This PR is auto-generated from #20121 to be assessed for backporting due to the inclusion of the label backport/1.13.x.

The below text is copied from the body of the original PR.

In the documentation,

vault auth enable userpass

vault write auth/userpass/users/mitchellh \
    password=foo \
    policies=admins

is used, which works but, there should a additional information about the custom path on the doc, or changes on the write command to clarify what is happening.

Propose addition:

Custom Path

vault auth enable userpass -path=custom_path

vault write auth/custom_path/users/mitchellh \
    password=foo \
    policies=admins