A few words fit the pattern well but are only found it very restricted combinations, or collocations. 'Piping' goes with 'voice', and 'gangling' goes with 'youth' or 'boy'. Since grammar mostly deals with generalities, we feel that it could be misleading to print them in lists which are intended to encourage compositon.
(Quoted from the introduction of Collins COBUILD English Grammar )

Is 'composition' referring to a piece of writing here?

If that is the case, how then would it be misleading to include words that have exclusive usages in lists?

Thank you

Composition here means the act of composing(writing) text.

I think they're saying that they don't wish to include words that a very narrow focus or narrow range of possibilities into a list of general or widely applied grammar rules intended to encourage writing.

In other words, some people might inappropriately try to use the narrow focus words in all situations. If those special usage cases are included on a general 'how to' grammar list, which applies to all situations, then those words which are reserved for special combinations or collocations could easily be confused as not being special usage or of narrow focus, and people might use them outside of that restricted focus.
Thanks for your reply, Skrej.
I think understand what they were trying to get across now. By the way, what I really meant to say was something along the lines of 'creating a piece of writing' instead of the seemly nonsense 'a piece of writing'. Have to do some 'second guessing' !