This looks good. Here are the results of my scrutinization; all are minor editorial issues.
"Sets and bags (also known as multisets) are unordered collections that can contain any Scheme object."
Actually any Scheme object _except comparators_, right?
May a multi-values object be an element? If so there are some minor picky edits, for example "The value that mapper returns is added to the set" should be "The object that mapper returns is added to the set".
"Sets are a standard part of the libraries of many high-level programming languages..."
You might want to cite SRFI 1 list sets here too.
"and can be optimized" sounds awkward, consider rewording to "and allows for optimization".
"hysterical raisins" should be "historical reasons" right?
"Implementations must not require the provision of both a comparison procedure and a hash function."
Any reason for this? An implementor might want to use a hybrid hash/tree data structure such as a treap.
"Note that this differs from the SRFI 1 analogue because it does not return a useful value."