Document the 'experimental.connpool' module

[andreyaksenov]

Documented a new experimental.connpool module.
This PR fixes the following doc issues:

• the experimental.connpool module #4129
• The mode option for call() and filter() #4151
• The instances, replicasets and groups options #4156

Staging: https://docs.d.tarantool.io/en/doc/connpool-api/reference/reference_lua/connpool/

Note

The code samples used in this document are created using the sharded_cluster_crud_docker demo application.

[lenkis]

Suggested change

	An ``experimental.connpool`` module provides a set of features for connecting to remote cluster instances and for executing remote procedure calls on an instance that satisfies the specified criteria.
	The ``experimental.connpool`` module provides a set of features for connecting to remote cluster instances and for executing remote procedure calls on an instance that satisfies the specified criteria.

[lenkis]

I see that elsewhere in the doc we keep using a definite article before module names, even when introduced for the first time (e.g. https://docs.d.tarantool.io/en/doc/connpool-api/reference/reference_lua/iconv/), with the only exception of explicitly adding "new" like in "a new XXX module" in release notes, but not in the main doc.

[lenkis]

Suggested change

	You need to grant the ``execute`` :ref:`permission <configuration_credentials_managing_users_roles_granting_privileges>` for the specified function to a user used for replication to execute this function on a remote instance.
	You need to grant the ``execute`` :ref:`permission <configuration_credentials_managing_users_roles_granting_privileges>` for the specified function to users with the ``replication`` :ref:`role <box_space-user>` to execute this function on a remote instance.

[andreyaksenov]

Technically speaking, a user used for replication and a user with the replication role might be different entities. In particular, user used for replication maintains inter-cluster communications while a user with the replication role can be created to allow some third-party system to get data from a Tarantool cluster (for example, CDC).

[lenkis]

Suggested change

	Once you have a connection, you can execute requests on a remote instance, for example, select data from a space using :ref:`conn.space.\<space-name\>:select() <conn-select>`.
	Once you have a connection, you can execute requests on the remote instance, for example, select data from a space using :ref:`conn.space.\<space-name\>:select() <conn-select>`.

[lenkis]

Meaning "the instance to which the connection is established". As I understand, within a single connect() function call a connection can be established to only 1 instance, so the instance is unique in this context.