adding accidental to "tr" for trill spanner

[Paul Scott]

This is probably simple but some parts of the wonderful Lilypond 
documentation are not clear to me.

Rather than use a pitched trill I need the other version of a trill 
where the "tr" is followed by an accidental.

Can someone direct to the documentation where I can change the "tr" to 
either "trb" or "tr#"?

Thank you,

Paul