8260694: (fc) Clarify FileChannel.transferFrom to better describe "no bytes available" case

Reviewed-by: alanb

Author: Hamlin Li

src/java.base/share/classes/java/nio/channels/FileChannel.java

@@ -642,7 +642,8 @@ public abstract long transferTo(long position, long count,
      * number of bytes will be transferred if the source channel has fewer than
      * {@code count} bytes remaining, or if the source channel is non-blocking
      * and has fewer than {@code count} bytes immediately available in its
-     * input buffer.
+     * input buffer. No bytes are transferred, and zero is returned, if the
+     * source has reached end-of-stream.
      *
      * <p> This method does not modify this channel's position.  If the given
      * position is greater than the file's current size then no bytes are