Implicit type conversion -- where documented?

    select 3.1416 || '?';

and the number implicitly converts to text and concatenates just fine, but writing:

   select trim(3.1416);

fails with an error message.  This seems odd to me--in both cases a float literal is used in a context expecting text; in one case it implicitly converts, in the other it doesn't.

*This brings up my real question*: are the details of this documented anywhere?  Chapter 10 refers to ' implicit conversions' but I can't see anywhere that the docs explain the details of how it is done, that would explain the observed difference in behavior described above.