Merge pull request #897 from stan-dev/fix-822

mention that exponential uses inverse scale (rate)

Author: bob-carpenter

src/functions-reference/positive_continuous_distributions.qmd

@@ -293,7 +293,7 @@ quantities blocks. For a description of argument and return types, see section
 
 ### Probability density function
 
-If $\beta \in \mathbb{R}^+$, then for $y \in \mathbb{R}^+$, \begin{equation*}
+If inverse scale (rate) $\beta \in \mathbb{R}^+$, then for $y \in \mathbb{R}^+$, \begin{equation*}
 \text{Exponential}(y|\beta) = \beta \, \exp ( - \beta \, y ) . \end{equation*}
 
 ### Distribution statement