Document new 'config' API

[andreyaksenov]

Updated reference for the config module. Fixed doc issues:

• config: access configuration of other cluster members #4105
• instance_uri() method for config module #4120
• Expansion of config:info() #4094

Staging:

• A new opts argument in config:get() (in particular, opts.instance)
• A new version argument in config:info()
• A new config:instance_uri() function
• A new config:instances() function

Note

New code examples are created using the sharded_cluster_crud_docker demo application.

[lenkis]

Suggested change

	Note that there is the following difference between getting a configuration for the current and remote instance:
	Note the following differences between getting a configuration for the current and remote instance:

[lenkis]

Suggested change

	- For the current instance, ``get()`` returns an instance configuration considering :ref:`environment variables <configuration_environment_variable>`.
	- For the current instance, ``get()`` returns instance configuration considering :ref:`environment variables <configuration_environment_variable>`.

[lenkis]

Suggested change

	- For a remote instance, ``get()`` only considers a cluster configuration and ignores environment variables.
	- For a remote instance, ``get()`` only considers cluster configuration and ignores environment variables.

[lenkis]

Suggested change

	The ``alerts`` section informs that privileges to the ``bands`` space for ``sampleuser`` cannot be granted because the ``bands`` space has not created yet:
	The ``alerts`` section informs that privileges to the ``bands`` space for ``sampleuser`` cannot be granted because the ``bands`` space has not been created yet:

[lenkis]

Suggested change

	:param string uri_type: a URI type. There are the following URI types are supported:
	:param string uri_type: a URI type. The following URI types are supported:

[lenkis]

Suggested change

	In this example, the same actions are performed for instances from the specific replica set:
	In this example, the same actions are performed for instances from a specific replica set: