don't use full allocation on network drives (on windows)

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@9989 a83610d8-ad2a-0410-a6ab-fc0612d85776
add documentation on streaming implementation

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@9990 a83610d8-ad2a-0410-a6ab-fc0612d85776