Added more words about rejecting binary f-strings.

pep-0498.txt

@@ -459,7 +459,18 @@ combine 'f' with 'b' string literals. The primary problem is that an
 object's __format__() method may return Unicode data that is not
 compatible with a bytes string.
 
-#XXX: maybe allow this, but encode the output as ascii?
+Binary f-strings would first require a solution for
+bytes.format(). This idea has been proposed in the past, most recently
+in PEP 461 [#]_. The discussions of such a feature usually suggest either
+
+  - adding a method such as __bformat__() so an object can control how
+    it is converted to bytes, or
+
+  - having bytes.format() not be as general purpose or extensible as
+    str.format().
+
+Both of these remain as options in the future, if such functionality
+is desired.
 
 !s, !r, and !s are redundant
 ----------------------------
@@ -567,6 +578,9 @@ References
 .. [#] Differences in str.format() and f-string expressions
        (https://mail.python.org/pipermail/python-ideas/2015-July/034726.html)
 
+.. [#] PEP 461 rejects bytes.format(), see section "Proposed variations"
+       (http://legacy.python.org/dev/peps/pep-0461)
+
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