http-proxy-exceptions is not documented

[Neil Mayhew]

There needs to be an item for "http-proxy-exceptions" in the list of
Configuration Options in the "Customizing Your Subversion Experience"
chapter. It needs to say something like this:

This specifies a comma-separated list of patterns for repository
hostnames that should accessed directly, without using the proxy
machine. The pattern syntax is the same as is used in the Unix shell
(aka "globbing"). A repository hostname matching any of these patterns
will not be proxied.