Explain why we ignore reason-phrase.

Fixes #60.

draft-ietf-httpbis-messaging-latest.xml

@@ -754,10 +754,14 @@ https://www.example.org
 </artwork></figure>
 <t>
    The reason-phrase element exists for the sole purpose of providing a
-   textual description associated with the numeric status code, mostly
-   out of deference to earlier Internet application protocols that were more
-   frequently used with interactive text clients. A client &SHOULD; ignore
-   the reason-phrase content.
+   textual description associated with the numeric status code, mostly out of
+   deference to earlier Internet application protocols that were more
+   frequently used with interactive text clients.
+</t>
+<t>
+   A client &SHOULD; ignore the reason-phrase content because it is not a
+   reliable channel for information (it might be discarded or overwritten by
+   intermediaries, and it is not transmitted in other versions of HTTP).
 </t>
 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="reason-phrase"/>
   <x:ref>reason-phrase</x:ref>  = *( <x:ref>HTAB</x:ref> / <x:ref>SP</x:ref> / <x:ref>VCHAR</x:ref> / <x:ref>obs-text</x:ref> )
@@ -3267,6 +3271,10 @@ Upgrade: HTTP/2.0
   conflicts with the use of ranks in the <x:ref>TE</x:ref> header field.
   (<xref target="transfer.coding.registry"/>)
 </t>
+<t>
+   Explain why the reason phrase should be ignored by clients. (<xref
+   target="status.line"/>)
+</t>
 </section>
 </section>