misc/py-serverfiles: Update pkg-descr

The e-mail to the upstream maintainer is undeliverable, so
I wrote it myself.

Reported by:	mat

misc/py-serverfiles/pkg-descr

@@ -1,4 +1,5 @@
 An utility that accesses files on a HTTP server and stores them locally
-for reuse.
+for reuse. It caches remote content locally so that subsequent invocations
+return it from the local cache.
 
 WWW: https://pypi.org/project/serverfiles/