Specify that Reader::read does not block if data is available.

[mahkoh]

The details of this probably depend on the Reader implementation, i.e., what does it mean that data is available.

Some examples:

• A BufferedReader will not call read on the contained Reader if the buffer is not empty
• If reading from a file descriptor succeeds but not enough data is returned to fill the buffer, the Reader will not attempt a second read.

If the user wants the whole buffer to be filled (unless we're at EOF), he can use the try_fill method which is being introduced in #18059.

[steveklabnik]

It would see that this is implementation-specific, yeah. @aturon @alexcrichton is this kind of thing something to specify? /cc #21070

[alexcrichton]

The current documentation of Read::read states:

This function does not provide any guarantees about whether it blocks waiting for data

The Read trait has become much more general and I believe this issue is moot now.