@RalfJung pointed out in a [comment] that the previous phrasing of the
sentence can read like it is giving guarantees about free constant
definitions always undergoing CTFE, even when unused. That seems to be
how the compiler behaves right now, but it's unclear whether it's
intentional.

Be more precise and don't talk about free constants at all.

[comment]: rust-lang#1120 (comment)

This makes the example a bit bigger, but makes the reason why constants don't
immediately undergo CTFE more conspicuous. When there is computation in the
definition that use generic parameters, there is not enough information to
fully evaluate the definition eagerly.

I made it a `compile_fail` example so the run button on the page gives reader
useful outputs.