adjacent_filtered's postcondition doesn't correctly describe behaviour

I think the documentation for `adjacent_filtered`'s [postcondition](https://www.boost.org/doc/libs/1_68_0/libs/range/doc/html/range/reference/adaptors/reference/adjacent_filtered.html#range.reference.adaptors.reference.adjacent_filtered), ie:

> For all adjacent elements `[x,y]` in the returned range, `bi_pred(x,y)` is `true`.

…isn't quite right. For example, this:

~~~cpp
#include

#include
#include

int main() {
const std::vector a = { 0, 1, 2, 3, 4, 5, 6 };

const auto b = a | boost::adaptors::adjacent_filtered(
[] (const int &x, const int &y) {
return ( y % 2 == 1 ) && ( y == x + 1 );
}
);
for (const auto &x : b) {
std::cerr << x << "\n";
}
}
~~~

…outputs:

~~~no-highlight
0
1
3
5
~~~

Yet two of the pairs of adjacent elements `[x,y]` in this range fail `bi_pred(x,y)` (because they differ by 2, not 1).

I think it's more like: `bi_pred` is true on each element in the returned range preceded by the element that preceded it in the original range (not in the returned range).