Add remaining non encoded descriptions

[ehwenk]

• There were 154 nutrient content traits where there was never a "non-encoded" description. For the new traits table we'll put on Zenodo, I think it will be good for all traits to have a "plain English" description. The descriptions added are identical to those in the encoded descriptions, except without the references to published classes. This was mostly done with

  mutate(
  description = ifelse(is.na(description), description_encoded, description),
  description = stringr::str_replace_all(description, "[:space:]\\[[:alnum:]+[:punct:][:alnum:]+\\]", "")
  )

• Also, a few bits of non-standard punctuation in the APD_categorical_values.csv, which became apparent when I tried to programmatically separate synonyms into their own column

[dfalster]

I've approved the PR, but note that it's very hard to see all the changes, as they appear in blocks where the entire line has changed

so I wasn't able to review these changes in detail