login.conf: document how to specify env var values with commas

As of f32db406504e quotes may be used to specify login class
capabilities that include commas. This is true in general but is
particularly relevant for setenv, a comma-separated list of environment
variables and values, so mention it there.

PR: 236204
Sponsored by: The FreeBSD Foundation

(cherry picked from commit ab2f6ae8d1742f57552c37425e9cab8499d2d4ea)