JFR: Remove Javadoc warnings

The following javadoc issues have been found in the jdk.jfr module:

src/jdk.jfr/share/classes/jdk/jfr/consumer/RecordedObject.java:257: warning: no comment
    protected Object objectAt(int index) {
                     ^
src/jdk.jfr/share/classes/jdk/jfr/EventType.java:73: warning: no @param for name
    public ValueDescriptor getField(String name) {
                           ^
src/jdk.jfr/share/classes/jdk/jfr/internal/Control.java:69: warning: no comment
    public abstract String combine(Set<String> values);
                           ^
/src/jdk.jfr/share/classes/jdk/jfr/internal/Control.java:73: warning: no comment
    public abstract void setValue(String value);
                         ^
src/jdk.jfr/share/classes/jdk/jfr/internal/Control.java:77: warning: no comment
    public abstract String getValue();
                           ^