www/{lib,}nghttp2: Improve descriptions

Make clear that www/libhttp2 is a HTTP/2 library with several consumers and
www/nghttp2 is a toolset using that lib. www/nghttp2 does not provide any C
library by itself.

See also: https://reviews.freebsd.org/D49114

Approved by: jrm (mentor), sunpoet, fluffy (maintainer timeout)
Differential Revision: https://reviews.freebsd.org/D49118