[spec] GH #909 Update the socket doc for read and recv for our new POSIX behaviour

Before parrot-4.10 the high-level read() returned the wanted amount of bytes.
Now it behaves the same as recv(), which means that up to bytes are returned

We might want to add a setsockopt(level, option, value) method.
[GH #909] Add test for chunked socket read

Socket.read will return chunked parts.
Merge branch 'rurban/socket-gh909'
Add api.yaml entry for GH #909 Socket.read(>512)