RFR: JDK-8295914: Add a header to generated HTML files in specs

[Jonathan Gibbons]

Please review some updates to the parts of the build system related to building the documentation.

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Commit messages:
 - JDK-8295914: Add a header to generated HTML files in specs

Changes: https://git.openjdk.org/jdk/pull/10891/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10891&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295914
  Stats: 144 lines in 3 files changed: 139 ins; 1 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/10891.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10891/head:pull/10891

PR: https://git.openjdk.org/jdk/pull/10891