Updates to correspondence display key

[kmcmullen]

Text marked with <unclear> currently displays as grey font with a "[?]". I think that's fine, but we should explain what it means in the display key. We may want to consider whether it's too similar to the archivist note display...?

[kmcmullen]

We also need to account for metamarks in the key (these are the marks with blue shading that we originally created for the galley proofs). You can see one here: https://cdrhdev1.unl.edu/chesnutt/item/ccda.corr00152.

I think we can label them as "Proofreading mark"

[kmcmullen]

I have added metamark/proofreading mark to the display key for correspondence on the dev branch:

chesnutt/app/views/items/_show_correspondence_key.html.erb

Line 43 in cf744d8

<td>proofreading mark</td>

. We'll just want to make sure this gets updated on production when we do our next push.

[kmcmullen]

The thing about unclear text still stand; I didn't tackle that yet