add info on population_age arg in age-struct-pop vignette

vignettes/age-struct-pop.Rmd

@@ -26,7 +26,7 @@ This vignette describes how to simulate a line list with either a uniform or non
 
 The {simulist} package uses a [branching process](https://epiverse-trace.github.io/bpmodels/reference/chain_sim.html) which is independent of population age structure to simulate the line list, and then {simulist} _paints_ the demographic information onto the infected individuals (and in the case of `sim_outbreak()` or `sim_contacts()` the contacts of infected individuals). In other words, once the cases in the line list have been simulated the ages are assigned to each individual post hoc, specified by the age range, and age structure, if supplied.
 
-The default age structure in {simulist} is a uniform structure between a lower and upper age range. The `sim_linelist()` function (and other `sim_*()` functions) can accept a `<data.frame>` instead of the `numeric` vector to specify the age structure for specified age groups. This feature can be especially useful when wanting to simulate an outbreak in a region with a heavily non-uniform age structure, [for example younger populations such as Nigeria, or older populations such as Japan](https://ourworldindata.org/age-structure). 
+The age range and age structure for the simulation functions (`sim_*()`) in {simulist} is controlled by the `population_age` argument. The default age structure in {simulist} is a uniform structure between a lower and upper age range. The `sim_linelist()` function (and other `sim_*()` functions) can accept a `<data.frame>` instead of the `numeric` vector to specify the age structure for specified age groups. This feature can be especially useful when wanting to simulate an outbreak in a region with a heavily non-uniform age structure, [for example younger populations such as Nigeria, or older populations such as Japan](https://ourworldindata.org/age-structure). 
 
 ```{r setup}
 library(simulist)