Give a single figure multiple captions in rmarkdown pdf_book output

I am writing my thesis with bookdown. I want to include a List of Figures, but I don't want to include the whole of each figure's caption. My thought was to give each figure one caption that appears in the lof and a second caption that doesn't. I have tried to do this using fig.subcap but this doesn't give a second caption, I presume because it needs a second figure to exist.

In the example below I want to create a figure that has the caption "My Main Caption. Here is more detail", but in the lof just has: "My Main Caption".

---
title: "Queries"
header-includes:
- \usepackage{subfig}
output: 
  bookdown::pdf_book:
    toc: true
lof: yes
---


```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = FALSE)
library(ggplot2)


```{r irisfig, fig.cap='My Main Caption. Here is more detail'}
ggplot2::ggplot(iris, aes(Sepal.Length, Sepal.Width))+
  geom_point()

This doesn't work:

```{r irisfig2, fig.cap='My Main Caption.', fig.subcap= c('Here is more detail')}
ggplot2::ggplot(iris, aes(Sepal.Length, Sepal.Width))+
  geom_point()

The LaTeX command \caption offers an optional argument to specify a shorter caption for the toc, in the form \caption[Short caption]{Long caption}. This option is accessible through fig.scap in the header:

```{r irisfig2, echo=F, fig.cap='My Main Caption PLUS More Detail', fig.scap='My Main Caption', out.extra=''}
ggplot2::ggplot(iris, aes(Sepal.Length, Sepal.Width))+
  geom_point()
```

Which yields latex code:

\begin{figure}
\includegraphics{test9_files/figure-latex/irisfig2-1} 
\caption[My Main Caption]{My Main Caption PLUS More Detail}\label{fig:irisfig2}
\end{figure}