RFR: JDK-8220202: Simplify/standardize method naming for HtmlTree

[Jonathan Gibbons]

Please review a conceptually simple changeset to rename 3 methods on the 
internal HtmlTree class.

`addAttr` is renamed to `put` and two overloads of `addContent` are 
renamed to `add`.

All the changes to the files in src/jdk.javadoc were performed 
mechanically, with an IDE rename.

One test file needed to be updated as well; within that file, the edits 
were done with "find and replace" in a text editor.

Although simple, the number of use sites is large, making it a big 
changeset. It may be easiest to scan through the overall patch than to 
read the changes in each file.

-- Jon

JBS: https://bugs.openjdk.java.net/browse/JDK-8220202
Webrev: http://cr.openjdk.java.net/~jjg/8220202/webrev.00/