Summary
-------

Describe more explicitly how `ExecutableElement .getReceiverType()` is expected to behave when class files are the backing store.

Problem
-------

The receiver *parameter* mechanism was added as part of the type annotations feature. The language feature is estimated to not be widely used.

Many executables have a sensibly defined receiver *type* even if an executable does not happen to be declared with a receiver *parameter*.

The specification and reference implementation of `ExecutableElement .getReceiverType()` has gone through a few iterations, see JDK-8225755 in JDK 14.

The changes in this CSR are:

 - to require the behavior of returning a receiver type when annotations
   on a receiver type are found in a class file, the behavioral updates
   of JDK-8319196.
 - allow, but not require, a non-sentinel receiver type object to be returned when a receiver *parameter* is not defined

The second point allows non-reference implementations more flexibility in how to implement this method; the `javac` reference implementation will continue to return a sentinel "not present" object when a receiver parameter is absent, even from source code.

Solution
--------

Add a paragraph to `ExecutableElement .getReceiverType()` describing the behavior of the method after the improvements made under JDK-8319196.

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

    diff --git a/src/java.compiler/share/classes/javax/lang/model/element/ExecutableElement.java b/src/java.compiler/share/classes/javax/lang/model/element/ExecutableElement.java
    index d4c9c881f3b..8bff3748394 100644
    --- a/src/java.compiler/share/classes/javax/lang/model/element/ExecutableElement.java
    +++ b/src/java.compiler/share/classes/javax/lang/model/element/ExecutableElement.java
    @@ -89,6 +89,17 @@ public interface ExecutableElement extends Element, Parameterizable {
          * non-inner class, or an initializer (static or instance), has no
          * receiver type.
          *
    +     * <p>The receiver <em>parameter</em> is a syntactic device added
    +     * to the language for the purpose of hosting annotations. Even
    +     * when source code is used as the basis for creating an
    +     * executable, if a receiver parameter is not present in the
    +     * source code, an implementation may elect to return a {@code
    +     * NoType} object even in cases where a receiver <em>type</em> is
    +     * nominally defined on the executable in question, such as an
    +     * instance method.  When a receiver parameter is present and
    +     * hosting annotations, a suitably annotated receiver type is
    +     * returned.
    +     *
          * @return the receiver type of this executable
          * @since 1.8
          *