docs(aws-lambda): drop stale webm rejection from validateConfig docblock

PR #952 review nit (Miguel): the validateConfig.ts file-header comment
still claimed the SDK rejects webm, but the runtime check no longer
does (ALLOWED_FORMATS now includes 'webm'). Update the docblock to
reflect that only force-hdr remains an SDK-side rejection.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: jrusso1020

packages/aws-lambda/src/sdk/validateConfig.ts

@@ -12,8 +12,9 @@
  *
  * The check is deliberately narrow — it covers the *shape* errors any
  * caller could have surfaced with `tsc` if they passed a literal, plus
- * the documented `webm`/`force-hdr` rejections from §5.3 of the
- * distributed-rendering plan. Anything deeper (font availability, plan
+ * the `force-hdr` rejection (HDR mp4 isn't supported in distributed
+ * mode). webm was previously rejected here too; v0.7+ supports it via
+ * closed-GOP concat-copy. Anything deeper (font availability, plan
  * size cap, GPU mode at runtime) needs the actual planner.
  */