Clarify docs regarding remote services in catalog.xml:service/base

[drf5n]

https://www.unidata.ucar.edu/software/thredds/v4.6/tds/reference/Services.html#tdsServiceElemRequirements indicates that THREDDS catalog service elements' base attribute is only relative to the server head. However, absolute, remote bases do work. For instance:

<service name="wcs" serviceType="WCS" base="/thredds/wcs/"/>
<service name="wms" serviceType="WMS" base="https://otherhost.domain.org/thredds/wms/"/>
<service name="iso" serviceType="ISO" base="https://samehost.domain.org:7002/thredds/iso/"/>
<service name="ncml" serviceType="NCML" base="/thredds/ncml/"/>
<service name="uddc" serviceType="UDDC" base="/thredds/uddc/"/>

Could pass the ISO service off on a docker container running on the same host, and the WMS service off onto a different machine.

Documenting this could help folks if they want to separate loads across servers for different services, or experiment with other code or versions running on the same data.

(Reference Unidata ticket [THREDDS #NWX-395622]: remote services on https://www.unidata.ucar.edu/software/thredds/v4.6/tds/reference/Services.html )