transpose, transposition, and relative

[lilypond]

Something seems wrong here ... I'm trying to enter a part for Trombone in 
treble clef (ie in B flat).

Reading the manual, section 5.15.6 says "transpose" and "relative" don't work 
well together, but this doesn't make sense! "transpose" converts the pitch of 
music as it is *output*, while "relative" affects the pitch of music as it is 
*input*. Plus, I've been using both of these and the only problems I've noticed 
have been down to the idiot at the keyboard :-)

But I'm now trying to use "transposition" to *enter* music, and as far as I can 
tell it is simply being ignored. To give an example fragment ...

\version "2.4.0"
voiceTromboneI = \relative c' {
   \transposition bf
   bf2\ff bf | df df | c df | ef1 | bf2 bf | df df | c df | ef1 |
}

Whether the transposition directive is there or not seems to make no difference 
whatsoever to the music output. It should be shifting it by a tone. (By the 
way, I'm outputting on paper, not midi.)

Or should the "Bugs" thing in the manual have said that "relative" and 
"transposition" are incompatible - in which case it's a real pain in the neck 
because I've either got to enter all the notes in absolute pitch with the right 
number of 's and ,s, or I've got to transpose it to concert in my head before I 
enter it! If this is a bug, can we class it as serious?

Cheers,
Wol