Fix Fields doc reference to getMany() usage in v3.x

[mfaywu]

There's a statement at the end of the section on ReferenceFields [0] which says that ReferenceField will use /users?ids=[] to fetch source data:

Then react-admin renders the with a loader for the , fetches the API for the related users in one call (GET http://path.to.my.api/users?ids=[789,735]), and re-renders the list once the data arrives. This accelerates the rendering and minimizes network load.

However, the beginning of the section on ReferenceFields [1] says ReferenceField will use dataProvider.getMany() to fetch source data:

This component fetches a referenced record (using the dataProvider.getMany() method), and passes it to its child.

The DataProvider page [2] shows that .getMany() is

GET http://my.api.url/posts?filter={"id":[123,456,789]}

So I think the statement at the end of the section on ReferenceFields [0] should be updated!

I'll open PRs for 1.x and 2.x as well since those look wrong too.

[0] https://marmelab.com/react-admin/doc/3.12/Fields.html#usage-9
[1] https://marmelab.com/react-admin/doc/3.12/Fields.html#referencefield
[2] https://marmelab.com/react-admin/doc/3.12/DataProviders.html#usage

[fzaninotto]

Well spotted, thanks!