Add note to error diagnostics when fallibility is due to function arguments

[jszwedko]

Users are frequently confused when a function marked is infallible in the docs is fallible because of the types of its arguments. The error message looks like:

  error[E103]: unhandled fallible assignment
    ┌─ :7:30
    │
  7 │         .splunk_sourcetype = downcase(type)
    │         -------------------- ^^^^^^^^^^^^^^
    │         │                    │
    │         │                    this expression is fallible
    │         │                    update the expression to be infallible
    │         or change this to an infallible assignment:
    │         .splunk_sourcetype, err = downcase(type)
    │
    = see documentation about error handling at https://errors.vrl.dev/#handling
    = learn more about error code 103 at https://errors.vrl.dev/103
    = see language documentation at https://vrl.dev
    = try your code in the VRL REPL, learn more at https://vrl.dev/examples

I think helpful if the diagnostics mentioned the fallibility is due to the type of the function arguments and, perhaps, included a link to https://vector.dev/docs/reference/vrl/expressions/#function-call-characteristics.

[pront]

One more follow-up on this: https://github.com/vectordotdev/vrl/pull/553/files#r1391630156