In the user written documentation section in the files here (at RC) there is a good document by Barry Gordon on the more common format (starting with 0000) that most signals use. I forget where I saw a more recent document on many of the oher formats (used in just a subset of the models). Maybe I can find a link to that tomorrow.

The 5000 codes are simpler than most:

5000 That's what tells that this is an RC-5
0072 In general, that word gives the wavelength of the modulating frequency (See Barry's document for details). I'm not certain, but I think the 5000 type actually ignores that and unconditionally uses the correct wavelength for RC-5.
0000 0001 Those give the counts of PAIRS of values in the two sections of following data. The first is for data used once at the start of a key press. The second is for data used repeatedly while the key is pressed. I believe the 5000 type has built in knowledge of the fact that the whole signal repeats, so I think using 0001 0000 instead of 0000 0001 would make no difference.
0014 003A are two numbers that get encoded into the signal actually sent. The rules for that are totally different for 5000 type vs. other types.