cntl: Clarification changes:

- Move IPV6 functionality to cntl-private.h. The library does not fully
  support ipv6 itself (we don't test for this), and neither does the
  cluster currently. Having this option in the main cntl.h file is
  confusing as it lets the user thing this actually works.

- Remove REINIT_DSN alias. This was used in some earlier beta versions
  of node.js before we renamed dsn to connstr

- Remove extraneous //-style comments (automatically inserted by my
  editor at times).

- Fix up SYNCDESTROY documentation to use common doxygen alias

Change-Id: I3846a21a80b076143a59ddd39f4bbc583e28d214
Reviewed-on: http://review.couchbase.org/39955
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>