adaptors: Documentation for sliced reports the wrong precondition

[Frederik Aalund <frederikaalund@…>]

Currently, the precondition for the sliced adaptor is

Precondition: 0 <= n && n <= m && m < distance(rng).

It should instead be

Precondition: 0 <= n && n <= m && m <= distance(rng).

Notice the <= instead of <.

The page in question is http://www.boost.org/doc/libs/1_58_0/libs/range/doc/html/range/reference/adaptors/reference/sliced.html.

Nitpicky: Maybe a better upper bound would be m <= size(rng) since boost::size returns a range_size<rng>::type which is the type of n and m (the parameters given to adaptors::sliced). Contrast this to boost::distance which returns a range_difference<rng>::type.