Mention second argument of log_mix can be array

Author: WardBrian

src/functions-reference/real-valued_basic_functions.qmd

@@ -1636,7 +1636,8 @@ proportion theta, defined by \begin{eqnarray*}
 Calculates the log mixture density given `thetas`,
 mixing proportions which should be between 0 and 1 and sum to 1,
 and `lps`, log densities.
-These two containers must have the same length.
+The `lps` variable must be either a 1-d container of the same
+length as `thetas`, or an array of such.
 
 \begin{eqnarray*}
 \mathrm{log\_mix}(\theta, \lambda)