DOC: config: unify "log" directive doc

DOC: config: unify "log" directive doc

"log" directive description was found 2 times in the configuration file:

First, in 3.1 in the "global parameters" chapter, and then in 4.2 in the
per-proxy keyword options.

Both descriptions are almost identical: having to maintain the "same"
documentation in 2 different places is error-prone. Due to this, some
precisions have been added in one of them, and were missing from
the other, and vice-versa, probably because one didn't see that the
"log" directive was also documented elsewhere.

To prevent the 2 descriptions from further diverging, and make it easier
to maintain, we merge them in the per-proxy "log" directive description
(in 4.2 chapter), and we add a pointer to it in the global "log" to
encourage the user to refer to the per-proxy "log" documentation for
usage details.