fix: add rule to omit doc-only changes from release notes

The release skill was inconsistently surfacing doc-comment-only changes
(e.g. Terraform description additions, docstring wording updates) as
user-facing features in some releases but not others. This led to
different release note content between Go and Python SDKs for the same
underlying config changes.

Add an explicit filter rule to Part 1 generation: skip changes that only
modify comments/docstrings/descriptions with no API behavioral change.

Author: algis-dumbris

.agents/skills/release/SKILL.md

@@ -153,6 +153,12 @@ We're excited to announce version {VERSION}!
   changes. Skip noise.
 - **Omit `codegen metadata` and `aggregated API specs update`** entries unless
   they introduce a specific user-visible change visible in the diff.
+- **Omit doc-only changes** — if a diff only updates comments, docstrings, or
+  descriptions (e.g., `"IPv4"` → `"IPv4 or IPv6"` in a docstring, or adding a
+  description to a service/resource for Terraform enablement) with no API
+  behavioral change (no new fields, methods, types, or changed signatures),
+  do **not** surface it in Part 1. These are internal metadata updates, not
+  user-facing SDK changes.
 
 Display the generated Part 1 to the user. Ask if they want to edit or approve.