[JAVA-1391] Javadoc for com.mongodb.connection package

Added missing Javadoc for this package.  There are still classes outstanding that require documentation, that need someone else's expertise:

 - AsyncCompletionHandler (2 missing comments)
 - ByteBufferOutputBuffer (2 missing comments)
 - NativeAuthenticationHelper.java (2 missing comments)
 - PowerOfTwoBufferPool (3 missing comments)
 - ResponseBuffers (3 missing comments)