Document objects with memory

[ThomasBreuer]

The GAP library contains the files lib/memory.gd and lib/memory.gi.
The functionality from these files is not used in the GAP library, and it is not documented.

Several packages use it and refer to it, for example the recog package, and it is often useful in interactive sessions.
Thus this functionality should be documented.
I can provide a pull request for that.

[ThomasBreuer]

This documentation should in particular state warnings:
It may happen that a function runs into an error when it is called with elements with memory (see for example #5381).
In such cases, probably just a method for elements with memory is missing, and perhaps some existing methods are installed with too weak requirements.