579: Update search docs r=ahmednfwela a=danFbach

# Pull Request

## Related issue
Fixes #577, references #578 

## What does this PR do?
- Updates docs to better reflect how results can be consumed.
- Some of the existing docs are wrong as `SearchResult<T> result = await index.SearchAsync(...);` is actually invalid since an explicit cast is required from `ISearchable<T>` to the type of result.
- There was no context showing that the result should be cast to either `SearchResult<T>` or `PaginatedResult<T>` in order to populate properties `EstimatedTotalHits` or `TotalHits` and `TotalPages` respectively.

## PR checklist
Please check if your PR fulfills the following requirements:
- [x] Does this PR fix an existing issue, or have you listed the changes applied in the PR description (and why they are needed)?
- [x] Have you read the contributing guidelines?
- [x] Have you made sure that the title is accurate and descriptive of the changes?

If you have any critiques or anything you want added to the docs for this purpose, let me know. Just hoping to clarify how transform the interface correctly.

Technically the proper type could be derive by one of two methods:
```c#
var result = (SearchResult<T>)await index.SearchAsync<T>(...);
```
or
```c#
var result = await index.SearchAsync<T>(...);
if (result is SearchResult<T> searchResult)
{
    //...
}
```
I prefer the latter, so that's how i changed the docs.

Co-authored-by: Dan Fehrenbach <dan@fbach.us>
Co-authored-by: Clémentine <clementine@meilisearch.com>