RFR: JDK-8027413: Clarify javadoc for j.l.a.Target and j.l.a.ElementType

[Joel Borggren-Franck]

Hi

Please review this javadoc clarification for j.l.annnotation.Target and
j.l.annotation.ElementType as part of the type annotations work.

Webrev: http://cr.openjdk.java.net/~jfranck/8027413/webrev.00/
JBS:    https://bugs.openjdk.java.net/browse/JDK-8027413

This is based on the update to JLS from Alex:
http://cr.openjdk.java.net/~abuckley/308.pdf (section 1.6).

cheers
/Joel