Replace INI setting documentation with HDF settings documentation

[fredemmott]

• gives errors for typos
• much clearer for structures such as arrays and maps
• more familiar for FB engineers; this would make communication between FB and other users clearer
• INI support primarily exists because of a previous goal of being a drop-in replacement for PHP; this is no longer a goal

[fredemmott]

To clarify, HDF is already supported by HHVM - it's older than our INI file support. This is just a documentation change.

[jjergus]

much clearer for structures such as arrays and maps

I can't even find any description of the file format anywhere. The only thing I could find is https://en.wikipedia.org/wiki/Hierarchical_Data_Format which seems to be a completely different thing (some binary format? the whole documentation seems to be written with the assumption that it will only be written programatically)