Update docs to reflect that top-level updatable fragment spreads are allowed

Remove the "known type hole" bullet point from the website docs that stated
updatable fragments cannot be spread at the top level. Also update the code
comment in the validator to remove the outdated restriction.

Author: captbaritone

compiler/crates/relay-transforms/src/assignable_fragment_spread/validate_updatable_fragment_spread.rs

@@ -64,7 +64,6 @@ impl UpdatableFragmentSpread<'_> {
     /// Validate many conditions for spreads of updatable fragments:
     /// * the fragment spread contains no directives
     /// * there is no @if or @skip between the linked field and the fragment spread
-    /// * the fragment spread is not at the top level
     /// * the fragment's type is a superset or equal to the outer type
     ///   * this ensures that if we read this fragment with readUpdatableFragment,
     ///     the result is guaranteed to be valid, i.e. the concrete type is guaranteed

website/docs/guided-tour/updating-data/imperatively-modifying-store-data.mdx

@@ -269,7 +269,6 @@ function NameUpdater({ queryRef }: {
 
 * This particular example can be rewritten using `readUpdatableFragment`. However, you may prefer `readUpdatableQuery` for several reasons:
   * You do not have ready access to a fragment reference, e.g. if the call to `commitLocalUpdate` is not obviously associated with a component.
-  * You do not have ready access to a fragment where we select the **parent record** of the record we wish to modify (e.g. the `Query` in this example). Due to a known type hole in Relay, **updatable fragments cannot be spread at the top level.**
   * You wish to use variables in the updatable fragment. Currently, updatable fragments reuse the variables that were passed to the query. This means that you cannot, for example, have an updatable fragment with fragment-local variables and call `readUpdatableFragment` multiple times, each time passing different variables.
 
 <DocsRating />