RFR [15] JDK-8247815: doclint: recategorize "no description for ..." as MISSING, not SYNTAX

[Martin Buchholz]

Relatedly, today I noticed warnings for missing comments on non-public
elements with javadoc16 that did not appear in javadoc11.

CompletableFuture.java:1718: warning: no comment
    static final class AsyncSupply<T> extends ForkJoinTask<Void>
                 ^

Naturally I added the flag -Xdoclint:all,-missing
BUT I'd like to be able to control which access levels require javadoc comments.
Few style guides will want to require javadoc comments on private elements.