feat(docs): latest valid ancestor store methods (#2669)

**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.
<!-- Why does this pull request exist? What are its goals? -->

**Description**
* Add doc comments for `Store` methods `set_latest_valid_ancestor` &
`get_latest_valid_ancestor`
<!-- A clear and concise general description of the changes this PR
introduces -->

<!-- Link to issues: Resolves #111, Resolves #222 -->

Closes None

Author: fmoletta

crates/storage/store.rs

@@ -1074,13 +1074,16 @@ impl Store {
         self.engine.read_storage_snapshot(account_hash, start).await
     }
 
+    /// Fetches the latest valid ancestor for a block that was previously marked as invalid
+    /// Returns None if the block was never marked as invalid
     pub async fn get_latest_valid_ancestor(
         &self,
         block: BlockHash,
     ) -> Result<Option<BlockHash>, StoreError> {
         self.engine.get_latest_valid_ancestor(block).await
     }
 
+    /// Marks a block as invalid and sets its latest valid ancestor
     pub async fn set_latest_valid_ancestor(
         &self,
         bad_block: BlockHash,