document "A = ( 0*tensor::i + yscale( tensor::j ) + 0*tensor::k )" trick

[slayoo]

Migrated from: https://sourceforge.net/p/blitz/bugs/65/

As pointed out in this mailing list thread: https://sourceforge.net/p/blitz/mailman/message/32742056/

mixing arrays of different dimensionality in the example expression requires a trick employing placeholders like "0*tensor::i" etc - perhaps worth documenting.