RFR: [15,doc] JDK-8247382 : doclint errors (missing comments) in jdk.compiler and jdk.javadoc

[Jonathan Gibbons]

Please review a trivial doc-only change to fix 3 missing doc comments
detected by doclint. The new comments do not add any semantically
new or interesting information, so no CSR.

For the javac case, a preferable solution would have been to remove
the undocumented `throws Exception`, except that would be an
incompatible signature change for anyone unlikely enough to be calling
the method directly. For now, the `throws` is just documented, even
though in practice the exception will never be thrown.

-- Jon

JBS: https://bugs.openjdk.java.net/browse/JDK-8247382
Webrev: http://cr.openjdk.java.net/~jjg/8247382/webrev.00/