RFR: JDK-8157682: @inheritDoc doesn't work with @exception [v4]

[Jonathan Gibbons]

> Please review a fix to improve the handling of the legacy `@exception` tag.
> 
> A patch for this was [previously submitted](https://mail.openjdk.java.net/pipermail/javadoc-dev/2019-September/001139.html). However, that patch introduced an explicit new `ExceptionTaglet` class, whereas the fix here just reuses the existing `ThrowsTaglet`, registering it under an additional name.
> 
> The test in the original patch is enhanced to check all 4 combinations of (throws, exception) in the base class and (throws, exception) in the subclass.

Jonathan Gibbons has updated the pull request incrementally with one additional commit since the last revision:

  update comment for allTaglets

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2818/files
  - new: https://git.openjdk.java.net/jdk/pull/2818/files/51974ef2..daeb8570

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2818&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2818&range=02-03

  Stats: 6 lines in 1 file changed: 5 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2818.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2818/head:pull/2818

PR: https://git.openjdk.java.net/jdk/pull/2818