Individual properties can be documented by calling the description method of a PropertyDescriptor's builder as such:

public static final PropertyDescriptor MY_PROPERTY = new PropertyDescriptor.Builder()
 .name("My Property")
 .description("Description of the Property")
 ...
 .build();

If the property is to provide a set of allowable values, those values are presented to the user in a drop-down field in the UI. Each of those values can also be given a description:

public static final AllowableValue EXTENSIVE = new AllowableValue("Extensive", "Extensive", "Everything will be logged - use with caution!");
public static final AllowableValue VERBOSE = new AllowableValue("Verbose", "Verbose", "Quite a bit of logging will occur");
public static final AllowableValue REGULAR = new AllowableValue("Regular", "Regular", "Typical logging will occur");

public static final PropertyDescriptor LOG_LEVEL = new PropertyDescriptor.Builder()
 .name("Amount to Log")
 .description("How much the Processor should log")
 .allowableValues(REGULAR, VERBOSE, EXTENSIVE)
 .defaultValue(REGULAR.getValue())
 ...
 .build();