Updates to CLM50_Tech_Note_Fire.rst by Fang Li

[slevis-lmwg]

Specific notes

Contributors other than yourself, if any:
@lifang0209

CTSM issues resolved or otherwise addressed, if any:
Resolves #3871

Testing planned or performed, if any:

• @lifang0209 already built successfully, and I will repeat this myself before merging

Requirements before merge:

• I have followed the CTSM contribution guidelines.

[slevis-lmwg]

@lifang0209 two questions for you:

1. Do you mention that the parameter values differ for other versions of the model, such as coupled and hillslope? If not, do want to give me some wording that you will come up with?
2. Please see my copy of the built docs to confirm that you still like how things look (you can see anything I changed in my second, third, and fourth commits above):
   /glade/derecho/scratch/slevis/preview_docs_pr.ESCOMP.CTSM.pr-4043/ESCOMP-CTSM-d12321f/doc/_build/html