docs(l1): latest valid ancestor store methods

[fmoletta]

Motivation
The Store methods set_latest_valid_ancestor & get_latest_valid_ancestor can be confusing without proper documentation. These methods were properly documented on the StoreEngine trait, but they were not documented in the Store structure where they will be most often called from. This PR adds documentation for these methods on the Store implementation while also simplifying it, as the internal trait documentation provides more information on the context and design choices/requirements for the implementation which are not necessary for the top-level methods.

Description

• Add doc comments for Store methods set_latest_valid_ancestor & get_latest_valid_ancestor

Closes None