Update the description of `.snap` file format

[Gumix]

Related dev. issue(s): tarantool/tarantool#7954

Product: Tarantool
Root document: https://www.tarantool.io/en/doc/latest/dev_guide/internals/file_formats/#the-snapshot-file-format
SME: @ Gumix

Details

Change "the .snap file’s records are ordered by space id" to:

Since Tarantool 3.0 the .snap file's records have the following order:

1. System spaces (id >= 256 && id <= 511), ordered by id;
2. All other (non-system) spaces, ordered by id.

In older versions of Tarantool the order of non-system spaces is unspecified.

Requested in tarantool/tarantool@2c39ff7