Clarify usage of from and to in Data Index GraphQL endpoint documentation

[gciavarrini]

The Data Index GraphQL endpoint currently lists the following operators for Number and Date:

* between: Numeric range
* from: Integer
* to: Integer

and

* between: Date range
* from: Date time
* to: Date time

However, from and to are not standalone operators. They should only be used as part of the between operator.

To improve clarity, I propose updating the documentation as follows:

* `between`: Date range (with `from` and `to` values required)

and

* `between`: Numeric range (with `from` and `to` values required)

[ricardozanini]

@gciavarrini can you open a PR?

[domhanak]

Resolved by #680