clarify snapshot scheduler randomization logic

[nekufa]

right now there is no information about randomization in snapshot scheduling process.
i think that documentation should be extended

it's not clear that snapshot interval can be multiplied twice by randomization logic.
https://github.com/tarantool/tarantool/blob/master/src/box/checkpoint_schedule.c#L50

on manual snapshot next checkpoint is also rescheduled using randomize
tarantool/tarantool#4432

[lenkis]

Info about snapshooting is scattered over these topics:

https://www.tarantool.io/en/doc/2.3/book/admin/backups/

https://www.tarantool.io/en/doc/2.3/reference/configuration/#checkpoint-daemon

https://www.tarantool.io/en/doc/2.3/reference/reference_lua/box_snapshot/

Please decide where this info belongs best.

[nekufa]

@lenkis sorry for delay, was afk
i think that second link is correct - according to the text on this page, checkpoints should be created every checkpoint_interval seconds. this is misleading, because interval is randomized in real world and can be multiplied twice or be less then is configured