Fix a bunch of Javadoc problems

[dflemstr]

This does the following:

• Corrects Javadoc that's just plain wrong (i.e. Function5: accepts three arguments)
• Fixes all warnings emitted by mvn javadoc:javadoc (including missing parameter documentation)
• Adds {@link} and {@code} where it makes sense
• Adds package-level documentation
• Annotates methods with the version number that they were introduced