docs: update documentation for symbolic_container

docs/src/api.md

@@ -5,7 +5,6 @@
 ### Mandatory methods
 
 ```@docs
-symbolic_container
 is_variable
 variable_index
 variable_symbols
@@ -26,6 +25,10 @@ allvariables
 
 ### Optional Methods
 
+```@docs
+symbolic_container
+```
+
 #### Observed equation handling
 
 ```@docs

src/index_provider_interface.jl

@@ -5,6 +5,11 @@ Using `indp`, return an object that implements the index provider interface. In
 itself implements the interface, `indp` can be returned as-is. All index provider interface
 methods fall back to calling the same method on `symbolic_container(indp)`, so this may be
 used for trivial implementations of the interface that forward all calls to another object.
+
+Note that this method is optional. Thus the correct method to check for a fallback is:
+```julia
+hasmethod(symbolic_container, Tuple{typeof(indp)}) && symbolic_container(indp) != indp
+```
 """
 function symbolic_container end