Fix documentation for folly's to<double>() method's handling of 'NaN' and unparseable...

Fix documentation for folly's to<double>() method's handling of 'NaN' and unparseable inputs

Summary:
The documentation for folly::to<double>()'s handling of unparseable inputs (including "") is incorrect.  Additionally, the documentation does not document to<double>()'s handling of the inputs "NaN", "-NaN", "inf", "infinity", "-inf", and "-infinity".

This corrects the documentation.

See https://github.com/facebook/folly/blob/3272dfdbe243e0ac02acca1dd82bc391d1da079a/folly/docs/Conv.md under the heading "Parsing floating-point types" for the incorrect documentation as rendered.

Reviewed By: yfeldblum

Differential Revision: D5367656

fbshipit-source-id: 22b38e2bb85d2e647975798360ead39eed6caae8