When extra-content is a list, add 4 space indentation and a newline to every
line. If it's a string, continue inserting it directly. This makes the list
serialization behavior more consistent with other services.
* gnu/services/web (default-nginx-config): Support lists.
* doc/guix.texi (Web Services)[nginx-configuration]: Document it.
Change-Id: Iec8614ba3cfc37292a566197e8d39b352b04846a
Signed-off-by: Christopher Baines <mail@cbaines.net>