update downgrade help

ran into an issue

fix for long term issue: #915

tsent/cmd/downgrade.go

@@ -16,18 +16,24 @@ var downgradeInfo downgradeArgs
 var downgradeCmd = &cobra.Command{
 	Use:   "downgrade",
 	Short: "downgrade db",
-	Long: `This downgrades the database to the specified version. It also deletes the generated schema files. To keep the generated schema files, pass the --keep_schema_files argument.
+	Long: `This downgrades the database to the specified version. It also deletes the generated schema files. 
+	
+To keep the generated schema files, pass the --keep_schema_files argument.
+
 When there's a branch and there's multiple heads and you wanna downgrade one branch, the command is as follows:
 tsent downgrade rev@branchrev
 For example, 
 if a change is made in one branch: rev1 -> rev2a 
 and in another branch: rev1 -> rev2b 
 and you're in main and both have been upgraded and you want downgrade just rev2a: the command is tsent downgrade rev2a@rev1
+
+If you're trying to downgrade a single revision e.g. downgrade -- -1 and get "Ambiguous walk" and you end up with a situation where there's only one head and multiple down revs, try running tsent downgrade current@one_down_rev and see if that fixes it
 `,
 	Example: `tsent downgrade --keep_schema_files -- -1
 tsent downgrade --keep_schema_files revision
 tsent downgrade -- -1
-tsent downgrade revision`,
+tsent downgrade revision
+tsent downgrade rev2a@rev1`,
 	Args: cobra.MinimumNArgs(1),
 	RunE: func(cmd *cobra.Command, args []string) error {
 		// another hardcoded place