Added prefix and @prefix to docs

Author: torfjelde

docs/src/api.md

@@ -21,6 +21,13 @@ One can nest models and call another model inside the model function with [`@sub
 @returned_quantities
 ```
 
+In the context of nesting models, it's also useful to prefix the variables in sub-models to avoid variable names clashing:
+
+```@docs
+prefix(::DynamicPPL.Model, x)
+@prefix
+```
+
 ### Type
 
 A [`Model`](@ref) can be created by calling the model function, as defined by [`@model`](@ref).