Following the discussion about the problems of the documentation for rubyore 
available on www.ruby-doc.org, I've tried to understand exactly which of the 
files in the ruby source distribution contain classes/methods of the core. 
From my investigation, these files are:

numeric.c
gc.c
io.c
re.c
range.c
variable.c
dir.c
signal.c
marshal.c
error.c
compar.c
enum.c
file.c
eval.c
random.c
hash.c
math.c
pack.c
prec.c
time.c
process.c
object.c
sprintf.c
parse.y
array.c
version.c
string.c
bignum.c
struct.c
class.c

NOTE: the methods I used to select the files which belong to the core from 
those which don't are *extremely* naive, so there can be some missing or extra 
files (in particular, the following files may be missing: dln.c, dmydln.c, 
dmyext.c, inits.c, lex.c, main.c, parse.c, regex.c, ruby.c, st.c, util.c. I 
looked at them, but they didn't seem to contain ruby classes or methods, and 
no class in the generated documentation pointed to them, so I decided to 
remove them). 

To get the documentation for only the core of ruby, download the ruby source 
from http://www.ruby-lang.org/en/downloads/, extract it and run rdoc on the 
files mentioned above (they're only in the toplevel directory).

Stefano