IMHO the chmod documenation is fairly comprehensive.
"... The first element of the list must be the numeric mode, which should probably be an octal number, and which definitely should not be a string of octal digits: 0644 is okay, but "0644" is not. Returns the number of files successfully changed. See also oct if all you have is a string."
Note the link to oct, and the fact that the examples clearly show the leading zero. Then follows examples explaining the outcome of several ways of using chmod, including the outcome of using it without the leading zero incorrectly. If you feel this isn't clear enough feel free to contribute to the documentation.
Update: Strike out mistake.