sql: set explicit default collation's strength

Before the patch, collations with no strength set used
tertiary strength. But it was not easy to understand it,
because box.space._collation:select{} would return
... [1, 'unicode', 1, 'ICU', '', {}] ... for such collations.
After the patch default value is set explicitly, so
user would observe : ... [1, 'unicode', 1, 'ICU', '',
{strength='tertiary'}] ...

Closes #3573

@TarantoolBot document
Title: default collation strength is explicit tertiary now
Before the patch we already have tertiary strength is default
strength for collations, but it was implicit:
[1, 'unicode', 1, 'ICU', '', {}]
After the patch it's just become explicit:
1, 'unicode', 1, 'ICU', '', {'strength' = 'tertiary'}]

Also please fix this https://tarantool.io/en/doc/2.1/book/box/data_model/#collations
There is line saying: "unicode collation observes all weights,
from L1 to Ln (identical)" It was not true and now this fact
would just become obvious.