Fix inaccurate ServiceInstanceIdResourceProvider class Javadoc

[thswlsqls]

Fixes #8646

Description

• The class Javadoc claimed the provider "does not implement ResourceProvider", which is false: it implements ConditionalResourceProvider, and ConditionalResourceProvider extends ResourceProvider.
• The old text was also a subjectless sentence fragment.
• Rewrote it to explain the provider uses ConditionalResourceProvider rather than a plain ResourceProvider because it runs after the other providers have discovered their attributes.
• Javadoc-only; no code, signature, or behavior change.

Testing done

• Docs-only, test-exempt: no test added. ./gradlew :sdk-extensions:incubator:check passed (spotless + ErrorProne/NullAway + jApiCmp + tests).
• No apidiff change (alpha module, Javadoc-only).

[opentelemetry-pr-dashboard]

Pull request dashboard status

Status last refreshed: 2026-07-20 21:23:43 UTC.

• Waiting on: Author
• Next step: Move out of draft to request review.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.64%. Comparing base (09d6c17) to head (542b9d3).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8647   +/-   ##
=========================================
  Coverage     91.64%   91.64%           
  Complexity    10348    10348           
=========================================
  Files          1013     1013           
  Lines         27380    27380           
  Branches       3218     3218           
=========================================
  Hits          25092    25092           
  Misses         1558     1558           
  Partials        730      730           

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