Document initialize

[jpellegrini]

Hi @egallesio !

I needed the CLOS initialize-instance method and couldn't find it, then I found that STklos implements it with the name "initialize"... And also found that it wasn't documented yet, so I have written some documentation. Is the explanation I gave OK?

[egallesio]

This is perfect.
I have just placed the documentation with make and finally renamed initialize as initialize-instance (since its friend is called allocate-instance). Of course, initialize is still usable.

I have also added the definition of make at the end, of your explanation to explicit the relation between make, allocate-instance, and initialize-instance.

Thanks a lot for this PR.