Replace the glossary entry for override inference

[bwilkerson]

The diagnostic messages documentation (https://dart.dev/tools/diagnostic-messages) contains a glossary entry for override inference. The text reads more like expository text than a glossary entry. I think we'll need to have documentation for Null Safety that explains how type inference in general, and override inference in particular, works, so it might be good to replace the glossary entry with a pointer to the inference documentation (after it exists).

[bwilkerson]

@domesticmouse

[domesticmouse]

I'm happy to help refine, but I lack the knowledge to write the original text. Help?

[bwilkerson]

(I probably should have said, but the referenced page hasn't actually been updated yet. The glossary entry is visible in this PR: #2655.)

I'm willing to expand on what I wrote for the glossary entry, but @munificent could probably do a better job, both with override inference and with other forms of inference.

[atsansone]

Updated in #2662. Closing.

[parlough]

I'm going to reopen this one. That just introduced the glossary entry. The idea here is to instead link to proper documentation on override inference (and type inference in general) when we finally get to expanding on that.

Related issues: #4745