Merge pull request SciML#3743 from AayushSabharwal/as/doc-changes

docs: fix FAQ docs, improve analysis point error message

Author: ChrisRackauckas

docs/src/basics/FAQ.md

@@ -27,7 +27,7 @@ are similarly undocumented. Following is the list of behaviors that should be re
     parameter with the given index.
   - `setindex!(::MTKParameters, value, ::ParameterIndex)` can be used to set the value of a
     parameter with the given index.
-  - `parameter_values(sys, sym)` will return a `ParameterIndex` object if `sys` has been
+  - `parameter_index(sys, sym)` will return a `ParameterIndex` object if `sys` has been
     `complete`d (through `mtkcompile`, `complete` or `@mtkcompile`).
   - `copy(::MTKParameters)` is defined and duplicates the parameter object, including the
     memory used by the underlying buffers.

src/systems/analysis_points.jl

@@ -348,7 +348,12 @@ function modify_nested_subsystem(
     end
     # ignore the name of the root
     if nameof(root) != hierarchy[1]
-        error("The name of the root system $(nameof(root)) must be included in the name passed to `modify_nested_subsystem`")
+        error("""
+        Invalid analysis point name `$(join(hierarchy, NAMESPACE_SEPARATOR))`. The name
+        must include the name of the root system `$(nameof(root))`. This typically happens
+        when  using an analysis point obtained by calling `getproperty` on a system marked
+        as `complete` to linearize a system that is not marked as `complete`.
+        """)
     end
     hierarchy = @view hierarchy[2:end]