man: document that too strict system call filters may affect the service manager

man: document that too strict system call filters may affect the service manager

If execve() or socket() is filtered the service manager might get into trouble
executing the service binary, or handling any failures when this fails. Mention
this in the documentation.

The other option would be to implicitly whitelist all system calls that are
required for these codepaths. However, that appears less than desirable as this
would mean socket() and many related calls have to be whitelisted
unconditionally. As writing system call filters requires a certain level of
expertise anyway it sounds like the better option to simply document these
issues and suggest that the user disables system call filters in the service
temporarily in order to debug any such failures.

See: #3993.