.Net: Update migration guide to cover new filtering API and update docs with API surface changes

[markwallace-microsoft]

[markwallace-microsoft]

Topic to consider:

We have a VectorSearchOptions type, but so does the the Mongo client SDK... Our type is generic, so (AFAIK) we can't do a using specifically for the VectorSearchOptions type to disambiguate; so instead I aliased the entire namespace. If you have a better suggestion am definitely interested!

BTW this will - to an extent - be annoying for real users of the Mongo connector. Fortunately, if they use target-typed new it's not a problem, since the type name can simply be omitted:

_ = await collection.VectorizedSearchAsync(vector, new() { /* options */ });
... but if they don't use target-typed new, users will have to do the same namespace aliasing done here. @westey-m @markwallace-microsoft this is probably a wrinkle that we should document...

[markwallace-microsoft]

Already completed for the March release