RFR: 8346289: Confusing phrasing in IR Framework README / User-defined Regexes

[theoweidmannoracle]

> If such a user-defined regex represents a not yet supported C2 IR node, it is highly encouraged to directly add a new IR node placeholder string definition to IRNode for it instead together with a static regex mapping block.

The combination of "instead together" makes this sentence hard to read. I had to re-read it several times to grasp it.

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Commit messages:
 - 8346289: Confusing phrasing in IR Framework README / User-defined Regexes

Changes: https://git.openjdk.org/jdk/pull/22766/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22766&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8346289
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/22766.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22766/head:pull/22766

PR: https://git.openjdk.org/jdk/pull/22766