Add ```box.schema.user.enable``` and ```box.schema.user.disable``` methods description to **Submodule box.schema**

[AArdeev]

Product: Tarantool
Root document: https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_schema/
SME: @ tekl1s

Details

• Currently, the box.schema.user.enable and box.schema.user.disable methods are merely mentioned in the documentation as notes in articles for box.schema.user.grant and box.schema.user.revoke.
• Both methods are useful tools for customers in terms of user management:

box.schema.user.enable('{username}') can be used instead of

box.schema.user.grant('{username}','usage,session','universe',nil,
{if_not_exists=true})

box.schema.user.disable('{username}') can be used instead of

box.schema.user.revoke('{username}','usage,session','universe',nil,
{if_exists=true})

• Both methods must be documented as separate sub-pages next to other APIs.

Example of use:

box.schema.user.enable (username)
---

box.schema.user.disable (username)
---