Update phenology tech note

[linniahawkins]

Description of changes

Added Leah Birch 2021 updates to boreal and arctic seasonal deciduous phenology onset and offset triggers.

Fixed some errors and revised phenology documentation for clarity.

Specific notes

Contributors other than yourself, if any:

CTSM Issues Fixed (include github issue #): #3867

Are answers expected to change (and if so in what way)? no

Any User Interface Changes (namelist or namelist defaults changes)? no

Does this create a need to change or add documentation? Did you do so? no/yes

Testing performed, if any: Checked rendering, looked good.

[samsrabin]

@linniahawkins the git-fleximod test failure here is because the PR updates FATES but not the corresponding entry in .gitmodules. I'm assuming that's unintended, so to resolve, (1) make sure your git status has no uncommitted changes and then (2):

bin/git-fleximod update fates
git add fates
git commit -m "Revert accidental FATES update"

[linniahawkins]

Shoot - okay I think I understand what happened. I'll try again.

[linniahawkins]

Okay I think I was able to fix it, thanks

[slevis-lmwg]

@linniahawkins thank you for your edits and updates in this section of the Technote. I have a few comments and a few requests for changes. I realize that I'm raising preexisting issues in some cases, but I think it's worthwhile fixing them.

[slevis-lmwg]

@linniahawkins thank you for your careful work in this PR. I had not realized how much extra work I was requesting with my review. Your PR could serve as an example for what we should all do to update our Sections.

I'm approving, and I will subsequently merge to b4b-dev.

[linniahawkins]

Thanks @slevis-lmwg - Very excited to have my first PR merged!