update branch description

Author: devalog

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

@@ -680,7 +680,7 @@ There are three ways (`mode`) to deploy your SDK code to GitHub:
 * **Push** - Fern generates your code and pushes it to the branch you specify. 
 
 For all of these modes, you must specify the GitHub repository in which you want to store your SDK code. You can optionally
-specify a branch name, license, and reviewers. 
+specify a branch name (required for `mode: push`), license, and reviewers. 
 
 ```yml {6-17}
 groups: 
@@ -709,6 +709,7 @@ groups:
 </ParamField>
 
 <ParamField path="branch" type="string" required={false} toc={true}>
+Required for `mode: push`.
 </ParamField>
 
 <ParamField path="license" type="'MIT' | 'Apache-2.0' | { custom: 'Custom License Name' }" required={false} toc={true}>