Fix documentation of BOOST_NETWORK_ENABLE_HTTPS

Fixes #429

Addresses comments by @povilasb on improvements to previous PR that
documents the circumstances where BOOST_NETWORK_ENABLE_HTTPS would be
set and be useful.
Regenerate updated documentation
Merge pull request #434 from deanberris/0.11-devel-docfix-https-refinement

0.11 devel docfix https refinement