www/nginx-full: Add maintainer note per new ports policy

The maintainer load and port lifecycle policy only excludes a port from the
outdated ratio when the reason is documented in the port itself:

https://wiki.freebsd.org/Ports/MaintainerLoadAndPortLifecyclePolicy

This is a slave port of www/nginx and follows the nginx stable branch, while
mainline is packaged as www/nginx-devel. repology merges the nginx ports into a
single project and measures the flavored slave ports against mainline: at the
time of writing it reports 1.30.4 here as outdated against 1.31.4, while the
unflavored www/nginx carrying the very same 1.30.4 is not reported at all.

No functional change, comment only.

Sponsored by: Netzkommune GmbH