docs: add "subject to change" warning to --viper-output

We insert a warning into the manual to let people know that the exact Viper
output is subject to change in the future.

Signed-off-by: Zoltan A. Kocsis <11808286+zaklogician@users.noreply.github.com>

Author: zaklogician

docs/manual.md

@@ -484,7 +484,8 @@ should be the same version and compiled with the same configuration options.
 
 If the `--viper-output PREFIX` argument is set, then for each protection domain `name` specified in
 the system description file, a file `PREFIX/name.vpr` will be output, containing a description of the
-capability table of the given PD in the Viper verification language.
+capability table of the given PD in the Viper verification language. These output files can be used
+for verification purposes, but the exact format may change in future versions of the tool.
 
 
 ## Image format