DOC: Fix "errorfile" description in the configuration manual

"errorfile" description is outdated. Now, error messages are parsed at startup
and converted to HTX messages. So they must be valid according to the HTTP
standards. In addition, there is now a comment about the reserved buffer space
that should remain available to allow header rewrites.

doc/configuration.txt

@@ -3793,17 +3793,15 @@ errorfile <code> <file>
 
   Code 200 is emitted in response to requests matching a "monitor-uri" rule.
 
-  The files are returned verbatim on the TCP socket. This allows any trick such
-  as redirections to another URL or site, as well as tricks to clean cookies,
-  force enable or disable caching, etc... The package provides default error
-  files returning the same contents as default errors.
-
-  The files should not exceed the configured buffer size (BUFSIZE), which
-  generally is 8 or 16 kB, otherwise they will be truncated. It is also wise
-  not to put any reference to local contents (e.g. images) in order to avoid
-  loops between the client and HAProxy when all servers are down, causing an
-  error to be returned instead of an image. For better HTTP compliance, it is
-  recommended that all header lines end with CR-LF and not LF alone.
+  The files are parsed when HAProxy starts and must be valid according to the
+  HTTP specification. They should not exceed the configured buffer size
+  (BUFSIZE), which generally is 16 kB, otherwise an internal error will be
+  returned. It is also wise not to put any reference to local contents
+  (e.g. images) in order to avoid loops between the client and HAProxy when all
+  servers are down, causing an error to be returned instead of an
+  image. Finally, The response cannot exceed (tune.bufsize - tune.maxrewrite)
+  so that "http-after-response" rules still have room to operate (see
+  "tune.maxrewrite").
 
   The files are read at the same time as the configuration and kept in memory.
   For this reason, the errors continue to be returned even when the process is