BUG/MINOR: payload/htx: Ingore L6 sample fetches for HTX streams/checks

Use a L6 sample fetch on an HTX streams or a HTX health-check is meaningless
because data are not raw but structured. So now, these sample fetches fail
when called from an HTTP proxy. In addition, a warning has been added in the
configuration manual, at the begining of the L6 sample fetches section.

Note that req.len and res.len samples return the HTX data size instead of
failing. It is not accurate because it does not reflect the buffer size nor
the raw data length. But we keep it for backward compatibility purpose.
However it remains a bit strange to use it on an HTTP proxy.

This patch may be backported to all versions supporting the HTX, i.e as far
as 2.0. But the part about the health-checks is only valid for the 2.2 and
upper.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index f5d19f9..8c33a5c 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -18404,6 +18404,14 @@
 for example through TCP request content inspection. Please see the "tcp-request
 content" keyword for more detailed information on the subject.
 
+Warning : Following sample fetches are ignored if used from HTTP proxies. They
+          only deal with raw contents found in the buffers. On their side,
+          HTTTP proxies use structured content. Thus raw representation of
+          these data are meaningless. A warning is emitted if an ACL relies on
+          one of the following sample fetches. But it is not possible to detect
+          all invalid usage (for instance inside a log-format string or a
+          sample expression). So be careful.
+
 payload(<offset>,<length>) : binary (deprecated)
   This is an alias for "req.payload" when used in the context of a request (e.g.
   "stick on", "stick match"), and for "res.payload" when used in the context of