Describe change-file-permissions-at in more detail

[sunfishcode]

change-file-permissions-at is vague about what it precisely does. This is necessary to some degree, due to differences between host operating systems. But it should at least say what it does on Unix and probably also Windows.

My rough idea when I proposed this function was that on Unix it would follow the user's umask. The umask is commonly 02, which means readable would be world-readable, writeable would be user/group-writeable, and executable would be world-executable.