I recommend leaving the text as is, and possibly creating a new FAQ entry discussing the relationship between immutability and hashability (something that I consider to be an intermediate or advanced topic).

Other thought:

* The set discussion should remain parallel that for mappings (a few paragraphs) after.  That text also discusses immutability

* We've never had a user incident regarding this text, so there is no actual evidence that this current wording is confusing anyone.

* It is common for users to equate hashability with immutability, so I think the current wording is reasonable.  My experience with users indicate that "hashable" is more cryptic than "immutable" because the former implies a knowledge of how hash tables work.

* It's easy for us thinking we're helping by making precise distinctions but have the actual effect of making the docs more opaque.  That is why first-aid books say "bruise" instead of "subdermal hematoma" :-)

* The word "immutable" is a reasonable first approximation that doesn't require knowledge of hash table mechanics.  For the most part, it is how everyday users think about dict keys and set elements. 

* That approximation is useful because a fuller discussion would say that if __hash__ is defined, it should do so on fields that don't mutate.