RFR 8179304: Please review minor changes to doc comments

[Jonathan Gibbons]

Please review these minor changes to make the doc comments HTML5 compliant.

Mostly, the changes are about changing <tt> to <code> or {@code} tags.

There is one minor adjustment to the text content -- in TreeVisitor.java,
I adjusted the name visitXYZ to visitXyz in line with similar usage 
elsewhere.

JBS: https://bugs.openjdk.java.net/browse/JDK-8179304
Webrev: http://cr.openjdk.java.net/~jjg/8179304/webrev/

-- Jon