Please add better documentation about what does page cache means in the context of this library

[oryair1]

Hi,
You explain in the readme file that libxdc must receive a callback which helps to "request memory":

"To disassemble the target, a callback page_cache_fetch_fptrhas to be provided that allows libxdc to request memory"

This is not at all understandable at least for me. Do you mean the default page cache of the operation system? and if so, why do you need it? I would very appreciate if you insert detailed documentation about why you need it. The Intel PT decoder by intel (libipt) does not require that, so what's the bonus in here?