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

[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.

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Commit messages:
 - JDK-8157682: @inheritDoc doesn't work with @exception

Changes: https://git.openjdk.java.net/jdk/pull/2818/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2818&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8157682
  Stats: 131 lines in 2 files changed: 120 ins; 3 del; 8 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