[#1237] Improve facet.limit search option

A negative value should be allowed if you don't want to limit the facet
values, but the search params schema didn't allow it. Also mention in
the docs that this can be set via configuration and that the default is
50. Added some tests.
[#1237] Fix docstring
Merge pull request #1237 from okfn/1237-facet.limit_improvements

'facet.limit' in package_search wrongly handled