Doc patch, normalize search_path in index

Hi,

The attached patch (against git head)
normalizes "search_path" as the thing indexed
and uses a secondary index term to distinguish
the configuration parameter from the run-time
setting.

"search path" the concept remains distinguished
in the index from "search_path" the setting/config param.
It's hard to say whether it's useful to make this
distinction. From a practical perspective it's easy
for the eye to stop scanning when the indent
level changes and so fail to notice that both
"search path" and "search_path" are index
entries. At least the index is a
lot more tidy than before.

Regards,

Karl <kop(at)meme(dot)com>
Free Software: "You don't pay back, you pay forward."
-- Robert A. Heinlein