Doc: NR 5.5.4 - Modifying ties and slurs

[Neil Puttock]

Hi James,

There's nothing wrong with the following:

 However, the @code{tie-configuration} property of
@code{TieColumn} can be overridden to set start line and direction
of ties as required.

'tie-configuration *is* a property of TieColumn, but one that happens
not to be set by default (that's why you don't see it in the
documentation for TieColumn).

Cheers,
Neil