log: add changelog and doc for "fix matras_view::block_count overflow"

The issue is fixed in the `small' submodule by the following commit:

* matras: fix matras_view::block_count overflow

Closes #3594

NO_TEST=The test requires more than 64 GB of RAM.

@TarantoolBot document
Title: Document maximum number of tuples in hash index
Product: Tarantool
Root document: https://www.tarantool.io/en/doc/latest/book/box/limitations/

Number of tuples in hash index: 2147483648