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

[Jonathan Gibbons]

Please review a one word change in doclint, and associated test updates,
to recategorize "no description for TAG" in the MISSING (-Xdoclint:missing)
category instead of the SYNTAX (-Xdoclint:syntax) category.

The proposed change is the result of being "surprised" all these messages
show up in the syntax category, when running doclint over all modules,
for each of its categories.

-- Jon

JBS: https://bugs.openjdk.java.net/browse/JDK-8247815
Webrev: http://cr.openjdk.java.net/~jjg/8247815/webrev.00/