west: sign.py: recommend separator -- tool_args always

In the following command, the first argument `for_rimage` is passed to
`rimage` whereas `--for west` goes to west.

```
west sign  for_rimage --for west
```

This is somehow valid but we really don't want anyone to do that.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

scripts/west_commands/sign.py

@@ -34,9 +34,11 @@
 
    west sign -t your_tool -- ARGS_FOR_YOUR_TOOL
 
-The "ARGS_FOR_YOUR_TOOL" value can be any additional
-arguments you want to pass to the tool, such as the location of a
-signing key etc.
+The "ARGS_FOR_YOUR_TOOL" value can be any additional arguments you want to
+pass to the tool, such as the location of a signing key etc. Depending on
+which sort of ARGS_FOR_YOUR_TOOLS you use, the `--` separator/sentinel may
+not always be required. To avoid ambiguity and having to find and
+understand POSIX 12.2 Guideline 10, always use `--`.
 
 See tool-specific help below for details.'''