DOC: Extend documentation for self_relative_ptr

[KFilipek]

DOC: Extend documentation for self_relative_ptr

Description

It would be good to add some info about limitations of self_relative_ptr (here and in the self_relative_ptr section if there is no info):

1. self-relative ptr which resides on pmem can only be used to point to elements in the same pool.
2. Adress of the element to which self-relative points must be aligned to 2 bytes

API Changes

N/A

Implementation details

Write example for that type of pointer.