Corrections to bgc documentation

[njeffery]

BFB

For detailed information about submitting Pull Requests (PRs) to the CICE-Consortium,
please refer to: https://github.com/CICE-Consortium/About-Us/wiki/Resource-Index#information-for-developers

PR checklist

• [ x] Short (1 sentence) summary of your PR:
  More updates and corrections to the zbgc documentation
• [ x] Developer(s):
  njeffery ; eclare ; apcraig
• Suggest PR reviewers from list in the column to the right.
  eclare/apcraig
• [ x] Please copy the PR test results link or provide a summary of testing completed below.
  viewed as html
• How much do the PR code changes differ from the unmodified code?
  • [x ] bit for bit
  • different at roundoff level
  • more substantial
• Does this PR create or have dependencies on CICE or any other models?
  • Yes
  • [x ] No
• Does this PR add any new test cases?
  • Yes
  • [ ]x No
• Is the documentation being updated? ("Documentation" includes information on the wiki or in the .rst files from doc/source/, which are used to create the online technical docs at https://readthedocs.org/projects/cice-consortium-cice/.)
  • [x ] Yes
  • No, does the documentation need to be updated at a later time?
    • Yes
    • No
• Please document the changes in detail, including why the changes are made. This will become part of the PR commit log.
  -Corrects a typos
  -Improves the notation for the iron equations
  -Corrects the DIC equation.

[apcraig]

Thanks Nicole! Keep them coming. I'll just keep merging them, we can review the generated documentation, and continue to revise as needed.