improve docs on make-weak-hash

Note that values are held normally and ephemerons can help.

pkgs/racket-doc/scribblings/reference/hashes.scrbl

@@ -170,7 +170,14 @@ See also @racket[make-custom-hash].}
 
 Like @racket[make-hash], @racket[make-hasheq], and
 @racket[make-hasheqv], but creates a mutable hash table that holds
-keys weakly.}
+keys weakly.
+
+Beware that values in the table are retained normally. If a value in
+the table refers back to its key, then the table will retain the value
+and therefore the key; the mapping will never be removed from the
+table even if the key becomes otherwise inaccessible. To avoid that
+problem, instead of mapping the key to the value, map the key to an
+@tech{ephemeron} that pairs the key and value.}
 
 @deftogether[(
 @defproc[(make-immutable-hash [assocs (listof pair?) null])