Integrated: 7903839: apidiff: Add a "make docs" target for the primary internal API

[Jonathan Gibbons]

Please review a small Makefile update to expose a friendly "make docs" target to generate API documentation for the internal API.

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Commit messages:
 - CODETOOLS-7903839: apidiff: Add a "make docs" target for the primary internal API

Changes: https://git.openjdk.org/apidiff/pull/14/files
  Webrev: https://webrevs.openjdk.org/?repo=apidiff&pr=14&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903839
  Stats: 4 lines in 2 files changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/apidiff/pull/14.diff
  Fetch: git fetch https://git.openjdk.org/apidiff.git pull/14/head:pull/14

PR: https://git.openjdk.org/apidiff/pull/14