[PATCH] troubleshooting http-pipelining for 1.9 release notes

Hi,

following patch adds the troubleshooting section to the 1.9 release
notes including a section about lock/unlock issues related to
old/ancient proxies.

I'd appreciate, if the actual description could be reviewed by someone
with a deeper knowledge on the underlying issue than I have.

[[[
Add a troubleshooting section to 1.9 to help users tracing down problems
related to proxies when locking/unlocking multiple files.

* docs/release-notes/1.9.html
   (troubleshooting): Add new section including http-pipelining issue
                      description.
]]]

Regards,
Stefan