[DOC] include ref guide updates for HBase 2.0 HBCK

Called out by stack in HBASE-19158

HBCK tool from an earlier release against an HBase 2.0+ cluster will destructively alter said cluster in unrecoverable ways.

Footnote or callout that says something like "Unfortunately we are unable to distinguish an HBCK client so cannot put in place guards against destructive HBCK changes....." .... probably too much for this startup section on reread of what I've written here.

As of HBase 2.0, HBCK is a read-only tool that can report the status of some non-public system internals. You should not rely on the format nor content of these internals to remain consistent across HBase releases.

Ugh. We need HBCK2 and a pointer here to it. Will do in a follow-on.

then update the upgrade section to point to the docs