Merge #344

344: fix docs for gray atmosphere optics r=szy21 a=szy21

<!--- THESE LINES ARE COMMENTED -->
## Purpose 
<!--- One sentence to describe the purpose of this PR, refer to any linked issues:
#14 -- this will link to issue 14
Closes #2 -- this will automatically close issue 2 on PR merge
-->


## To-do
<!---  list of proposed tasks in the PR, move to "Content" on completion 
- Proposed task
-->


## Content
<!---  specific tasks that are currently complete 
- Solution implemented
-->


<!---
Review checklist

I have:
- followed the codebase contribution guide: https://clima.github.io/ClimateMachine.jl/latest/Contributing/
- followed the style guide: https://clima.github.io/ClimateMachine.jl/latest/DevDocs/CodeStyle/
- followed the documentation policy: https://github.com/CliMA/policies/wiki/Documentation-Policy
- checked that this PR does not duplicate an open PR.

In the Content, I have included 
- relevant unit tests, and integration tests, 
- appropriate docstrings on all functions, structs, and modules, and included relevant documentation.

-->

----
- [ ] I have read and checked the items on the review checklist.


Co-authored-by: Zhaoyi Shen <11598433+szy21@users.noreply.github.com>

docs/src/Optics.md

@@ -7,8 +7,7 @@
 The gray atmosphere (more precisely, semi-gray atmosphere) approximates the radiative properties of the atmosphere by taking the optical properties to be independent of wavelength, separately in the longwave and shortwave bands. 
 
 ### Longwave
-Two options are currently supported for computing the optical depth for longwave radiation.
-The optical depth (``d``) for longwave radiation, with the "GrayOpticalThicknessSchneider2004" option, follows [schneider2004](@cite):
+Two options are currently supported for computing the optical depth for longwave radiation. The optical depth (``d``) for longwave radiation, with the "GrayOpticalThicknessSchneider2004" option, follows [schneider2004](@cite):
 
 ```math
 \begin{align}
@@ -43,10 +42,10 @@ d(\phi, p) = \alpha \left[f_l \sigma + (1 - f_l) \sigma^4 \right] \left[ \tau_e
 \end{align}
 ```
 
-where $f_l = 0.2$, $\sigma = \frac{p}{p_0}$ is pressure p normalized by
-surface pressure $p_0$, $\phi$ is latitude, and the longwave optical thicknesses at the equator and at the pole are  $\tau_e = 7.2$ and $\tau_p = 1.8$, respectively. $\alpha$ is a scaling factor. 
+where ``f_l = 0.2``, ``\sigma = p / p_0`` is pressure p normalized by
+surface pressure ``p_0``, ``\phi`` is latitude, and the longwave optical thicknesses at the equator and at the pole are  $\tau_e = 7.2$ and $\tau_p = 1.8$, respectively. $\alpha$ is a scaling factor. 
 
-The optical thickness of an atmosphere layer (the differential optical depth) of pressure thickness $\Delta 𝑝$ is
+The optical thickness of an atmosphere layer (the differential optical depth) of pressure thickness ``\Delta p`` is
 ```math
 \begin{align}
 \tau(\phi, p) = (\alpha * \frac{\Delta p}{p}) \left[f_l \sigma + 4 (1 - f_l) \sigma^4 \right] \left[ \tau_e + (\tau_p - \tau_e) \sin^2\phi \right]
@@ -57,7 +56,9 @@ The source function for longwave radiation is calculated as ``S = \sigma T^4 / \
 
 ### Shortwave
 
-The optical depth (``d``) for shortwave radiation follows [frierson2007](@cite) and [ogorman2008](@cite):
+Two options are currently supported for computing the optical depth for shortwave radiation. The optical depth (``d``) for shortwave radiation, with the "GrayOpticalThicknessSchneider2004" option, is 0.
+
+The optical depth (``d``) for shortwave radiation, with the "GrayOpticalThicknessOGorman2008" option, follows [frierson2007](@cite) and [ogorman2008](@cite):
 
 ```math
 \begin{align}