[jdk16] RFR: JDK-8258657: Doc build is broken by use of new language features [v2]

[Erik Joelsson]

> This patch changes how the docs-reference make target behaves to better support the requirements for it. This target is used to generate javadocs in a more stable way between releases, so that they those docs are better suited for generating diffs between releases. Currently we use the boot JDK to run javadoc for these, but this has shown to be problematic. This patch introduced a specific configure parameter for the JDK to use just for generating these docs. If not set, it will revert to the default interim javadoc, just like the main docs are built (but still with a separate set of parameters).
> 
> This fix needs to go into JDK 16 so that the docs-reference target there can be used to generate diffs for JDK 17.

Erik Joelsson has updated the pull request incrementally with one additional commit since the last revision:

  Update copyright years

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Changes:
  - all: https://git.openjdk.java.net/jdk16/pull/88/files
  - new: https://git.openjdk.java.net/jdk16/pull/88/files/521c460c..5e5e7f78

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk16&pr=88&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk16&pr=88&range=00-01

  Stats: 5 lines in 5 files changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk16/pull/88.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk16 pull/88/head:pull/88

PR: https://git.openjdk.java.net/jdk16/pull/88