Summary
-------

Clarify handling of invalid inputs by `Types.{unboxedType, getWildcardType}`.

Problem
-------

The existing exception case specification for these methods is terse and does not explicitly state cases which could expected to be exceptional.

Solution
--------

Add more detail on exceptional cases.

Specification
-------------

    diff --git a/src/java.compiler/share/classes/javax/lang/model/util/Types.java b/src/java.compiler/share/classes/javax/lang/model/util/Types.java
    index 266d63178ba..232e15a84bd 100644
    --- a/src/java.compiler/share/classes/javax/lang/model/util/Types.java
    +++ b/src/java.compiler/share/classes/javax/lang/model/util/Types.java
    @@ -50,6 +50,11 @@
      * <p><b>Compatibility Note:</b> Methods may be added to this interface
      * in future releases of the platform.
      *
    + * @apiNote
    + * In the reference implementation, handling {@linkplain ErrorType
    + * error types} generally does not cause an {@code
    + * IllegalArgumentException} from the methods in this interface.
    + *
      * @see javax.annotation.processing.ProcessingEnvironment#getTypeUtils
      * @since 1.6
      */
    @@ -198,8 +203,10 @@ public interface Types {
          *
          * @param t  the type to be unboxed
          * @return the type of an unboxed value of type {@code t}
    +     *
          * @throws IllegalArgumentException if the given type has no
    -     *          unboxing conversion
    +     *         unboxing conversion. Only types for the wrapper classes
    +     *         have an unboxing conversion.
          * @jls 5.1.8 Unboxing Conversion
          */
         PrimitiveType unboxedType(TypeMirror t);
    @@ -268,7 +275,10 @@ public interface Types {
          *
          * @param extendsBound  the extends (upper) bound, or {@code null} if none
          * @param superBound    the super (lower) bound, or {@code null} if none
    -     * @throws IllegalArgumentException if bounds are not valid
    +     *
    +     * @throws IllegalArgumentException if bounds are not valid. Invalid bounds
    +     * include all types that are not {@linkplain ReferenceType
    +     * reference types}.
          * @jls 4.5.1 Type Arguments of Parameterized Types
          */
         WildcardType getWildcardType(TypeMirror extendsBound,