Implicit type conversion -- where documented?

I came across what seems like anomalous behavior regarding
implicit conversion from a numeric type to text. You can write:

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.

Thanks!