JDK 9 RFR of JDK-8172531: Correct misstatements in javax.lang.model visitor documentation

[joe darcy]

Hello,

Please review the selection of corrections and improvements to the 
javadoc of the visitor type of javax.lang.model in:

     JDK-8172531: Correct misstatements in javax.lang.model visitor 
documentation
     http://cr.openjdk.java.net/~darcy/8172531.0/

Description and guide to the changes:

* For better readability, the notation "visitXYZ" is replaced with 
"visitXyz".

* The visitor interface types in javax.lang.model.{element, type} have a 
change in verb tense to describe what was and was not done in Java SE 8.

* In the utility visitors, a paragraph mentioning default methods is 
removed.

* In the Kind visitor classes, the discussion of the when the kind 
methods are called uses a correct name "visitXyz*As*Kind" rather than 
the incorrect "visitXyzKind".

Thanks,

-Joe