Add guidance around using unit prefix/suffixes to ensure uncommon unit abbreviations are spelled out in the question or input label

[yatesn]

We should add guidance around using unit prefix/suffixes to ensure uncommon unit abbreviations are spelled out in the question or input label

Example of guidance from GDS:

Prefixes and suffixes are useful when there’s a commonly understood symbol or abbreviation for the type of information you’re asking for. Do not rely on prefixes or suffixes alone, because screen readers will not read them out.

If you need a specific type of information, say so in the input label or hint text as well. For example, put ‘Cost, in pounds’ in the input label and use the ‘£’ symbol in the prefix.

Also, update all the examples to ensure the unit is spelt out in the label.

Originally posted by @jrbarnes9 in https://github.com/ONSdigital/design-system/discussions/279#discussioncomment-1042326

Linked to #1604

[jrbarnes9]

Dupe of #1604