Invalid requirements for insert in the documentation

[Documentation states](http://boostorg.github.io/hana/group__group-Sequence.html#gae22a1a184b1b2dd550fa4fa619bed2e9):

>it must also be true that `n < length(xs)`

So if I have a sequence of size 2, the only valid indexes are 0 and 1 then I wouldn't be able to insert a new element at all valid places. Insert means insert before, so it should be valid to insert before the end iterator (i.e. `length(xs)`) and the actual requirement should be:

>it must also be true that `n <= length(xs)`

I haven't fully tested, but the compiler accepts `n=length(xs)`, so for now I'm assuming that only the documentation is outdated.

Thank you for the excellent library.

Start with the linked Boost.Hana Sequence documentation and locate the requirements for insert. Confirm that insertion before the end position is supported, then update the stated bound from n < length(xs) to n <= length(xs); done means the documentation matches the accepted behavior.

Written by the indexing model from the issue text.