Some trait functions are now listed as "const fn" even for types without a const trait impl

[RalfJung]

Consider for instance these docs:

This looks like I can call ne in const context but not eq, which makes little sense.

Cc @rust-lang/project-const-traits (is that ping group still the right one for this?)

[fee1-dead]

@rustbot claim

[fmease]

(dupe of #143071)