doc inaccuracies and leftover FIXME

[cachebag]

minor doc cleanup:

• lib.rs module docs list NetworkManager under models ("Core Types" section, ~line 311-314), but it's only available at the crate root (nmrs::NetworkManager), not via nmrs::models
• typo in error.rs:173: "An error occured while parsing" → "occurred"
• leftover FIXME in builders/vpn.rs:235: // FIXME: surely, there must be a better way to do this