RFR: Add material about updating the CSR when a backout occurs. [v2]

[Joe Darcy]

On Wed, 9 Apr 2025 21:58:52 GMT, Stuart Marks <smarks at openjdk.org> wrote:

>> Also, add a small fix to the Makefile.
>
> Stuart Marks has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Keep paragraph as a single line (per Jesper).

Marked as reviewed by darcy (Committer).

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

PR Review: https://git.openjdk.org/guide/pull/148#pullrequestreview-2754900744