select with offset and count are now logarithmic in memtx tree

Product: Tarantool
Since: 3.3.0
Root document:

• https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_index/count/#lua-function.index_object.count
• https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_index/select/#lua-function.index_object.select

SME: @ mkostoevr

Details

Root document: https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_index/count/#lua-function.index_object.count

Iterate over an index, counting the number of tuples which match
the key-value.

This is no longer true for the memtx tree index, where count has a
logarithmic complexity (O(log(size)) where size is the amount of
tuples in the index) and it does not iterate over the index.

Root document: https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_index/select/#lua-function.index_object.select

offset – the number of tuples to skip (use this parameter carefully
when scanning large data sets).

The part about large datasets is not actual for the memtx tree index
now, since the complexity of select with offset there is logarithmic
too (it does not scan the space either).

Use the offset option carefully when scanning large data sets as it
linearly increases the number of scanned tuples and leads to a full
space scan. Instead, you can use the after and fetch_pos options.

Ditto.

Root document: https://www.tarantool.io/en/doc/latest/platform/engines/memtx_vinyl_diff/

count() function Takes a constant amount of time

Now it takes logarithmic time as mentioned above.
Requested by @mkostoevr in https://github.com/tarantool/tarantool/commit/421d36f2e144ae2ebe54a2e9ef1ede3278cd5c69.

Review the linked box_index.count, box_index.select, and memtx_vinyl_diff documentation pages. Update the statements about count() and select() offset complexity to reflect the memtx tree behavior described in the issue, and remove or revise the warnings that no longer apply. Done means all three references consistently describe the current complexity and scanning behavior.