Add a "Revoking grants" paragraph to documentation

[AArdeev]

Product: Tarantool
Audience/target: Users
Root document:

• https://www.tarantool.io/en/doc/latest/platform/connections_and_auth/credentials
  SME: @ totktonada

Details

• Few places in documents explain how to grant privileges to users, but they do not explain revoking these privileges. This looks like a consistency gap.
• For example, to take a previously granted privilege to a lua function away, specify the lua_call option without the previously allowed function name, or with an empty privileges array (further options may retain commented-out):

# grant privilege
credentials:
  users:
    alice:
      privileges:
        - permissions: [execute]
          lua_call: [my_func, my_func2]

# take away a privilege:
credentials:
  users:
    alice:
      privileges:
        - permissions: [execute]
          lua_call: [my_func]

# take away all previously granted privileges:
credentials:
  users:
    alice:
      privileges: []
#        - permissions: [execute]
#          lua_call: [my_func, my_func2]