RFR: Add material about updating the CSR when a backout occurs.

[Stuart Marks]

Also, add a small fix to the Makefile.

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Commit messages:
 - Add material about updating the CSR when a backout occurs.

Changes: https://git.openjdk.org/guide/pull/148/files
  Webrev: https://webrevs.openjdk.org/?repo=guide&pr=148&range=00
  Stats: 6 lines in 2 files changed: 5 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/guide/pull/148.diff
  Fetch: git fetch https://git.openjdk.org/guide.git pull/148/head:pull/148

PR: https://git.openjdk.org/guide/pull/148