document fork default branch behavior

Author: williammartin

pkg/cmd/pr/create/create.go

@@ -214,7 +214,8 @@ func NewCmdCreate(f *cmdutil.Factory, runF func(*CreateOptions) error) *cobra.Co
 			Upon success, the URL of the created pull request will be printed.
 
 			When the current branch isn't fully pushed to a git remote, a prompt will ask where
-			to push the branch and offer an option to fork the base repository. Use %[1]s--head%[1]s to
+			to push the branch and offer an option to fork the base repository. Any fork created this
+			way will only have the default branch of the upstream repository. Use %[1]s--head%[1]s to
 			explicitly skip any forking or pushing behavior.
 
 			%[1]s--head%[1]s supports %[1]s<user>:<branch>%[1]s syntax to select a head repo owned by %[1]s<user>%[1]s.