docs(sdks): describe branch support in release mode

Expand the 'branch' ParamField under release-mode github config to
explain that it retargets commits and tags to a non-default branch,
and mention the new optional behavior in the publishing-mode snippet.

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

Author: Swimburger

fern/products/sdks/reference/generators-yml-reference.mdx

@@ -851,7 +851,7 @@ Name of your repository in GitHub.
 </ParamField>
 
 <ParamField path="branch" type="string" required={false} toc={true}>
-Name of your branch in GitHub.
+If specified, Fern commits and tags the release on this branch instead of the default branch. The branch must already exist. Use this to publish versioned SDKs (e.g., `v1` and `v2`) from a single repo.
 </ParamField>
 
 <ParamField path="license" type="'MIT' | 'Apache-2.0' | 'Custom License Name'" required={false} toc={true}>

fern/products/sdks/snippets/github-publishing-mode.mdx

@@ -1,7 +1,7 @@
 
 Optionally set the mode to control how Fern handles SDK publishing:
 
-- `mode: release` (default): Fern generates code, commits to main, and tags a release automatically
+- `mode: release` (default): Fern generates code, commits to the default branch (or the `branch` you specify), and tags a release automatically
 - `mode: pull-request` (recommended):  Fern generates code and creates a PR for you to review before release
 - `mode: push`: Fern generates code and pushes to a branch you specify for you to review before release