Document boost::combine(range1, range2)

[Nathan Ridge]

The header boost/range/combine.hpp contains a function combine() that combines two ranges into a "zipped range" built on top of the Boost.Iterator library's zip_iterator.

This function is not used as a helper by anything else in Boost.Range, so I'm guessing it should be public API.

If that's the case, it should added to the documentation.

[Neil Groves]

Reimplemented the combine() function and introduced reference documentation. Comitted to the develop branch.

There is a general task to improve the tutorial and example sections of the documentation that shall improve this very basic documentation at a later date.