Document removal of port 5986

[wohali]

See apache/couchdb#1523 and apache/couchdb#2336 for details.

Be sure to link the documentation in the 3.0 release notes, too.

Validate all the URLs in those PRs and the release notes for functionality as they are documented.

[wohali]

Files that mention 5986:

cluster/databases.rst
cluster/nodes.rst
cluster/sharding.rst
config/auth.rst
config/http.rst
config/services.rst
setup/cluster.rst

and of course whatsnew/3.0.rst needs updating to remove the TODO.