RFR: 8306980: Generated docs should contain correct GPLv2 Legal Documents

[KIRIYAMA Takuya]

I modified the location from which javadoc copies some legal files to the generated documentation.  If --legal-notices　option is set to default or nothing is specified,, GPLv2 Legal Documents are copied from legal/java.base/ directory, such as LICENSE, ADDITIONAL_LICENSE_INFO and ASSEMBLY_EXCEPTION.

Would you please review this fix?

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Commit messages:
 - 8306980: Generated docs should contain correct GPLv2 Legal Documents

Changes: https://git.openjdk.org/jdk/pull/13686/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13686&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8306980
  Stats: 48 lines in 2 files changed: 29 ins; 0 del; 19 mod
  Patch: https://git.openjdk.org/jdk/pull/13686.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13686/head:pull/13686

PR: https://git.openjdk.org/jdk/pull/13686