connection: add keyfile documentation section for connection
Summary
Add connection setting section to keyfile documentation. This documentation can be checked with man 5 nm-settings-keyfile
Checklist
Please read https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/blob/main/CONTRIBUTING.md before opening the merge request. In particular, check that:
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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