Add description of vshard.enable/disable

[Serpentian]

• Add description of the automatic and manual router API's
  enabling/disabling

Resolves #3012

[patiencedaur]

Thanks for the patch! Most of my suggestions are language-related.
I haven't looked at the changes on the staging website yet because the deployment is currently broken. If it's okay with you, I'll take another look at the PR after the deployment is fixed.

[patiencedaur]

Suggested change

	.. NOTE::
	``vshard.router.enable()`` cannot be used for enabling the router
	API which was automatically disabled due to running configuration
	process.
	.. note::
	``vshard.router.enable()`` cannot be used for enabling a router
	API that was automatically disabled due to a running configuration
	process.

1. We use .. note:: now: https://www.tarantool.io/en/doc/latest/contributing/docs/markup/admonitions/
2. A running configuration process is router.cfg(), right? Let's mention this.

[patiencedaur]

Suggested change

	Manually restrict access to the router API. Disabling makes all
	its methods, excluding :ref:`vshard.router.cfg() <router_api-cfg>`,
	:ref:`vshard.router.new() <router_api-new>`,
	:ref:`vshard.router.enable() <router_api-enable>` and
	:ref:`vshard.router.disable() <router_api-disable>`, to throw a Lua
	error in case of invoking them. The error object has a name attribute
	equal to ``ROUTER_IS_DISABLED``.
	Manually restrict access to the router API. When the API is disabled,
	all its methods throw a Lua error,
	except :ref:`vshard.router.cfg() <router_api-cfg>`,
	:ref:`vshard.router.new() <router_api-new>`,
	:ref:`vshard.router.enable() <router_api-enable>` and
	:ref:`vshard.router.disable() <router_api-disable>`. The error object's name attribute is
	``ROUTER_IS_DISABLED``.

Russian is a wordy language, and English is not. Please try to use fewer words and simpler constructions whenever possible.

[patiencedaur]

Suggested change

	forcefully disabled until configuration is finished as accessing the
	forcefully disabled until the configuration is finished, as accessing the

[Serpentian]

Closed as a duplicate of #3025