connection: add keyfile documentation section for connection

Fernando Fernández Mancera requested to merge ff/keyfile_connection_docs into main


Add connection setting section to keyfile documentation. This documentation can be checked with man 5 nm-settings-keyfile


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  • the subject for all commits is concise and explicative
  • the message for all commits explains the reason for the change
  • the source is properly formatted
  • any relevant documentation is up to date
  • you have added unit tests if applicable

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