Document `box.malloc.info()`

[TarantoolBot]

box.malloc.info() is a function that returns a table with two fields:

tarantool> box.malloc.info()
---
- size: 2498560
  used: 1835726
...

The used value is the total amount of memory in bytes allocated by
Tarantool for internal data structures with malloc().

The size value is the total amount of memory in bytes allocated from
the system by the malloc() allocator. It may not be less than used.

The function may be used before box.cfg() is called.

Currently, the function is available only on Linux. On other systems it
reports zero memory usage ({size = 0, used = 0}).

Requested by @locker in tarantool/tarantool@d34a0cb.