(doc) outdated - note ignore unfound

On specific sources, mention `--ignore-unfound` is available to ignore
the packages that are not on those items.

src/chocolatey/infrastructure.app/commands/ChocolateyOutdatedCommand.cs

@@ -99,7 +99,8 @@ choco outdated
     choco outdated -s ""'https://somewhere/protected'"" -u user -p pass
 
 If you use `--source=https://somewhere/out/there`, it is 
- going to look for outdated packages only based on that source.
+ going to look for outdated packages only based on that source, so 
+ you may want to add `--ignore-unfound` to your options.
 
 NOTE: See scripting in the command reference (`choco -?`) for how to 
  write proper scripts and integrations.