RFR: 8372393: Document requirement for separate metallib installation with Xcode 26.1.1

[Galder Zamarreño]

Document the metal toolchain requirements starting with Xcode 26.

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Commit messages:
 - Document Metal installation with XCode 26

Changes: https://git.openjdk.org/jdk/pull/28489/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28489&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372393
  Stats: 9 lines in 2 files changed: 9 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/28489.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28489/head:pull/28489

PR: https://git.openjdk.org/jdk/pull/28489