RFR: JDK-8320941: Discuss receiver type handling

[Joe Darcy]

Please review this note to discuss when ExecutableElement returns a non-sentinel object for the receiver type.

(The PR was accidentally created from the wrong branch; the change to java.lang.Double can be ignored.)

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Commit messages:
 - JDK-8320941: Discuss receiver type handling
 - JDK-8320940: Fix typo in java.lang.Double

Changes: https://git.openjdk.org/jdk/pull/16874/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16874&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320941
  Stats: 12 lines in 2 files changed: 11 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/16874.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16874/head:pull/16874

PR: https://git.openjdk.org/jdk/pull/16874