Update IMPORTANT_NOTES.md regarding fates_parteh_mode

[slevis-lmwg]

Description of changes

Turns out I didn't need to do much in the User's Guide to update fates_parteh_mode:

• Setting fates_parteh_mode does not qualify as a "special case"
• We already link to the latest namelist_definition and namelist_defaults
• I found one mention of fates_parteh_mode that needed updating in IMPORTANT_NOTES.md
• This .md file was rendering as a hard-to-read single line, and with a few simple changes it now renders as a readable yellow page

Specific notes

Contributors other than yourself, if any:

CTSM issues resolved or otherwise addressed, if any:
Resolves #3957

Requirements before merge:

• I have followed the CTSM contribution guidelines.

[slevis-lmwg]

@adrifoster I'm asking for your review of this FATES-related docs PR. Notes:

• Only line 63 is relevant to the title of the PR.
• The rest are formatting changes to get the .md file to render as a readable page in the docs rather than as a hard-to-read single-line string.

You can build the docs in my PR using Sam R.'s new tool ./preview_docs_pr if you want to see what I mean.