Document upper and lower bounds of chunk size.

This is hardcoded in s/chunk.cpp:

   bool Chunk::setMaxChunkSizeSizeMB( int newMaxChunkSize ) {
        if ( newMaxChunkSize < 1 )
            return false;
        if ( newMaxChunkSize > 1024 )
            return false;
        MaxChunkSize = newMaxChunkSize * 1024 * 1024;
        return true;
    }