Fix inconsistencies in javadocs.

These changes were made with consistency with the rest of the project, and with Oracle's Java code conventions in mind:
 - <p> tags are now on their own new line, separating paragraphs, and without empty new lines above and/or below them.
 - Remove unnecessary new lines separating lists and <pre> tags.
 - Remove unnecessary <p> tags for leading paragraphs.
 - Add missing <p> tags.
 - Remove white space immediately following <p> tags.
 - Remove all unnecesary closing </p> tags.

Souyrce for Oracle's Javadoc conventions:
http://www.oracle.com/technetwork/java/javase/documentation/index-137868.html