docs: add description to branch parameter of createSiteBuild (#604)

Author: khendrikse

go/plumbing/operations/create_site_build_parameters.go

@@ -1236,7 +1236,10 @@ paths:
       description: 'Runs a build for a site. The build will be scheduled to run at the first opportunity, but it might not start immediately if insufficient account build capacity is available. Files for build could be also uploaded as a zipped site.'
       parameters:
         - name: branch
-          description: The branch to build; defaults to main branch
+          description: >-
+            If no branch is specified, it is treated as a production deploy
+            If a branch IS specified and matches the main branch, it is also production
+            If a branch is specified and doesn't match the main branch, it is a branch deploy
           required: false
           in: query
           type: string