RFR: JDK-8304689: Add hidden option to disable external spec page [v2]

[Jonathan Gibbons]

> Please review a change to introduce a hidden option to temporarily disable the "External Specifications" page while we add `@spec` tags, and until we have a critical mass of such tags.

Jonathan Gibbons has updated the pull request incrementally with one additional commit since the last revision:

  Update src/jdk.javadoc/share/classes/jdk/javadoc/internal/doclets/formats/html/HtmlOptions.java
  
  Co-authored-by: Andrey Turbanov <turbanoff at gmail.com>

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/13127/files
  - new: https://git.openjdk.org/jdk/pull/13127/files/d89e76a0..45585cc8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13127&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13127&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13127.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13127/head:pull/13127

PR: https://git.openjdk.org/jdk/pull/13127