docs: specify ThreadSafeRepository is not Send/Sync without "parallel"

gix/src/types.rs

@@ -149,6 +149,9 @@ pub struct Reference<'r> {
 /// Note that it clones itself so that it is empty, requiring the user to configure each clone separately, specifically
 /// and explicitly. This is to have the fastest-possible default configuration available by default, but allow
 /// those who experiment with workloads to get speed boosts of 2x or more.
+///
+/// Note: when built with `default-features = false`, this type is **not** `Send`.
+/// The minimal feature set to activate `Send` is `features = ["parallel"]`.
 pub struct Repository {
     /// A ref store with shared ownership (or the equivalent of it).
     pub refs: crate::RefStore,
@@ -182,6 +185,9 @@ pub struct Repository {
 /// it's merely meant to be able to exist in a `Sync` context.
 ///
 /// Note that it can also cheaply be cloned, and it will retain references to all contained resources.
+///
+/// Note: when built with `default-features = false`, this type is **not** `Send` or `Sync`.
+/// The minimal feature set to activate `Send + Sync` is `features = ["parallel"]`.
 #[derive(Clone)]
 pub struct ThreadSafeRepository {
     /// A store for references to point at objects