Clarify how to add services to pre-existing resource registrations

From https://support.opensciencegrid.org/public/tickets/2f0465dda314583de2cf9399a9e4e1dbff39ee7ea222258147c690fdd7a7d1b2:

Points in the documentation that I found confusing:

1) When I tried to follow the instructions under "Searching for
resources", I first did a search for "ruhex-squid.rutgers.edu", which
(as expected) was not found. However, a search for
"ruhex-osgce.rutgers.edu" did return a result, for the CE portion of
T3_US_Rutgers. It was not clear to me whether I should add information
on ruhex-squid to that entry, or try to create an entirely new entry.

<snip>

Let's clarify in our instructions that in such cases, the additional service can just be added to the pre-existing resource.