clarify uv and par comments and doc

[eclare108213]

• Short (1 sentence) summary of your PR:
  Clarify uv and par radiation descriptions in code comments and documentation.
• Developer(s):
  @eclare108213
• Suggest PR reviewers from list in the column to the right.
• Please copy the PR test results link or provide a summary of testing completed below.
  See updated documentation. All code changes are in comments.
• How much do the PR code changes differ from the unmodified code?
  • bit for bit
  • different at roundoff level
  • more substantial
• Does this PR create or have dependencies on CICE or any other models?
  • Yes
  • No
• Does this PR add any new test cases?
  • Yes
  • 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/.)
  • 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.

The word 'visible' is confusing in the comments for ultraviolet radiation bands and not needed for PAR. It was removed for the uv and PAR variables but left as-is for older code, which used the term in a general sense for wavelengths < 700 nm. Changed the uv comments to state '< 400 nm' and added some text to the docs.

Closes #548

[eclare108213]

@zhaobin74 please take a look at these changes to make sure they are correct. Thanks!

[eclare108213]

As noted in #548, this issue is a bit complicated due to the evolution of the shortwave parameterization over many years. For this PR, in addition to removing the word 'visible' I changed the uv comments to state '< 400 nm' and added some (hopefully) clarifying text to the docs.

Ready for review.

[zhaobin74]

This looks clear to me. Thanks, @eclare108213.