Clarify explanation for not displaying Carpentries curriculum materials in "what to display" section

### How could the content be improved?

> What to Display
> [...] In most situations we do not recommend displaying Carpentries curriculum materials to your learners while you teach.
>
> The visual environment in a workshop should be focused on exactly what you are teaching and should mirror, as closely as possible, exactly what you say. This is because keeping track of distracting and contradictory sensory information adds to cognitive load. The [split-attention effect](https://en.wikipedia.org/wiki/Split_attention_effect) describes the cognitive effort involved with trying to assemble information from different modalities. Learning is most effective when visual displays, text, and auditory information presented together are the same, with minimal distractions.

I found this section confusing, because sometimes trainers, especially novice ones like me, may be reading directly from the curriculum text! In this case, the suggestion to "mirror, as closely as possible, exactly what you say" and the note that "Learning is most effective when visual displays, text, and auditory information presented together are the same" would actually point in favor of displaying the curriculum text, since the alternative would be to have only auditory information without the matching visual.

I think there's an unspoken meaning here: don't display the curriculum *because it is very common not to speak the text exactly*. But if that's the argument, we should state it clearly here.

### Which part of the content does your suggestion apply to?

https://carpentries.github.io/instructor-training/instructor/05-memory.html

Start with the “What to Display” section at instructor/05-memory.html and read the surrounding guidance. Clarify the unstated rationale about displaying curriculum text versus speaking it verbatim, while preserving the advice about reducing distractions. Done means the recommendation and its connection to the split-attention explanation are unambiguous.

Written by the indexing model from the issue text.