[patch] : Add doc and minor fix to partial_sort_copy

[Paolo Carlini]

chris jefferson wrote:

> OK, here are my two recent patches bundled up into one. Designed to
> apply to so_7 (some of the documentation could usefully go back, I can
> either split that up and apply it, or it could all go back in a
> possible future uber-patch back to so_6) 

Applied, thanks! Yes, later we may consider adjusting the comments in
4_0-branch too.

Paolo.