Expand docs with batching example

[sabard]

Add a short doc on the use of BatchSQLAlchemyConnectionField with a brief comment. It's only documented in the test files, but should also be included in the documentation.

Additionally, can expand docs on sort keyword to include disabling and custom usage.

Originally posted by @palisadoes in https://github.com/graphql-python/graphene-sqlalchemy/issues/357#issuecomment-1447481010