langref: exemplify practical difference between !T and anyerror!T

[Des-Nerger]

Since anytype in Zig means type inference, people may be misled into thinking that anyerror also does mean type inference; but just for error types. And so, they may get an idea that !T is just an abbreviated and nicer way to write the "frequently used" anyerror!T combination.

I'm not suggesting changing the language itself here, but at least the reference should grab an attention to the distinction, give an example of where one of them could accomplish more than another, etc.

[Rexicon226]

related: #5893

[nektro]

have you noticed this confusion happen in the wild? not saying we shouldnt add this but my initial reaction was that its unlikely given that anytype is the only any thats not a discrete type/value