Add environment stacks to docstring

Author: IanButterworth

base/initdefs.jl

@@ -376,6 +376,11 @@ end
     active_manifest(project_file::AbstractString)
 
 Return the path of the active manifest file, or the manifest file that would be used for a given `project_file`.
+
+In a stacked environment (where multiple environments exist in the load path), this returns the manifest
+file for the primary (active) environment only, not the manifests from other environments in the stack.
+See the manual section on [Environment stacks](@ref) for more details on how stacked environments work.
+
 See [`Project environments`](@ref project-environments) for details on the difference between a project and a manifest, and the naming
 options and their priority in package loading.