RFR: 8372940: Update symbol data script references

[Nizar Benalla]

Update path to the generate symbols script, and expand the start of release doc.

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Commit messages:
 - doc updates

Changes: https://git.openjdk.org/jdk/pull/28606/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28606&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372940
  Stats: 3 lines in 3 files changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/28606.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28606/head:pull/28606

PR: https://git.openjdk.org/jdk/pull/28606