Some fixes for proccontrol(1) man page.

• Fix markup.
• Mention that process can only allow tracing for itself. This is already stated in procctl(2), but requiring knowledge of the syscall description is too much for the tool user.
• Clearly state that query mode only works for existing process.

Noted and reviewed by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 3 days