fix for Nim 1.2 + explain readInto EOF semantics

[akavel]

Fix an error in Nim 1.2 on Windows:

asynctools\asyncpipe.nim(260, 16) Error: undeclared identifier: 'PCustomOverlapped'

Also, it took me suuuuper long to find out how to detect that asyncpipe.readInto encountered an EOF on Windows... so I thought it could be nice to add an explicit explanation in the docs...