Document mutability of reference types

[smallhive]

Marked method which returning value must not be changed, because it leads to data malfunction. Left as is places where we create a new var and return it by pointer. Such var mutation is safe for our structs.

closes #187

[roman-khimov]

Needs to be rebased now.

[roman-khimov]

Conflicts.

[smallhive]

Rebased