clarify 'strict' behaviour in tap tests

[TarantoolBot]

Product: Tarantool
Since: product version
Audience/target: dev
Root document: https://www.tarantool.io/en/doc/latest/reference/reference_lua/tap/
SME: @ olegrok

---

Defined for root tap object strict mode will be the same for all
subtests.

Example:

t = require('tap').test('123')
t.strict = true

t:is_deeply({a = box.NULL}, {}) -- false

t:test('subtest', function(t)
    t:is_deeply({a = box.NULL}, {}) -- also false
end)

Requested by @olegrok in tarantool/tarantool@676582c.