docs: Update TRAMP config recommendations

[jidicula]

• Use %C token for ControlPath in case user@host:port is too long for ssh - see https://github.com/ansible/ansible/issues/11536#issuecomment-153030743 and https://en.wikibooks.org/wiki/OpenSSH/Cookbook/Multiplexing#Multiplexing_Options
• Fix code block and styling.
• Use nil rather than empty string for disabling tramp-use-ssh-controlmaster-options.
• Slightly more human-readable ControlPersist