The standard doclet currently provides the old (up to JDK8) diagnostics mechanism and supports use of doclint. The two are (currently) mutually exclusive; enabling doclint disables the old mechanism; not enabling doclint is supposed to fall back on the old mechanism.

Due to a bug in JDK9, the fallback mechanism is broken, such that the legacy mechanism is not enabled when doclint is not enabled.

Having two mechanisms is one too many, although is should be noted that neither completely supersedes the other.

We should figure out how to unify the mechanisms.

Design issues to address:
1. What is the significance of `-Xdoclint:none` with respect to detecting serious errors?
2. Some conditions may require the full doclet environment: what about when doclint is invoked from javac (i.e full doclet env not available)

Also for consideration, the kinds of issues that can be detected in doc comments:
* issues that do not lead to bad (non-conformant) docs ... e.g. missing tags
* issues don't affect the doclet directly but lead to bad (non-conformant) docs ... e.g. bad HTML in doc comments
* issues that affect the doclet ... e.g. bad/unresolvable references to program elements