Differentiate between "role" and "user" (login) in BigAnimal docs

[josh-heyer]

This needs to be done very carefully, if at all; prefer "role" if not specifically talking about login. See: #4321

[josh-heyer]

See also: #4879

[josh-heyer]

DJ suggests reserving "user" for contexts where we're describing the act of logging in / interactive usage (vs. a role that could be used for this purpose but isn't explicitly being done)

[josh-heyer]

New idea: write short-ish explainer for PostgreSQL user/role and link to it in places (BA docs...) where the usage would otherwise be ambiguous.