Remove the use of load(on:) in relations.md

[wojexe]

As I've noticed on a discord conversation, the load(on:) method is discouraged to be used and should be replaced with get(on:), which defaults to cache if possible.

[gwynne]

Great! Can you also add mention (and possibly an example) of get()'s reload parameter? (The full signature is get(reload:on:), with reload defaulting to false.)

[wojexe]

Actually there is a mention of the reload: parameter higher up, in the #get section. I just thought that get(on:) would be sufficient for this use case. But yeah, on the second thought, people might want to have their data reloaded during eager loading, so I'll include it. 😄

[0xTim]

LGTM - given the additional text we'll need translations for this so I'll remove the no-translation-needed label