This commit rewords the expect header bug notice to provide the precise
details for the bug arising. In particular, the bug does not impact any
request over 1024 bytes, but instead impacts any request with a body
that is sent in two requests, the first with an Expect: 100-continue
header. The size is irrelevant, and requests with bodies larger than
1024 bytes are okay as long as the Expect: 100-continue header is not
also sent.
Relates #19911