Document `box_slab_info` C API function

[TarantoolBot]

Since: Tarantool 3.3.0

Added the new C function box_slab_info that returns information about
the memory state depending on the passed argument box_slab_info_type.
The same information is available in Lua via box.slab.info().
Requested by @SEChudinov in tarantool/tarantool@bd510bb.