doc: clarify Cookie API usage (seanmonstar#2111)

`cookie_store` and `cookie_provider` both enable the cookie store
functionality; clarify the usage and potential for a custom store
being overwritten if both are used.

src/async_impl/client.rs

@@ -786,7 +786,11 @@ impl ClientBuilder {
     /// Cookies received in responses will be preserved and included in
     /// additional requests.
     ///
-    /// By default, no cookie store is used.
+    /// By default, no cookie store is used. Enabling the cookie store
+    /// with `cookie_store(true)` will set the store to a default implementation.
+    /// It is **not** necessary to call [cookie_store(true)](crate::ClientBuilder::cookie_store) if [cookie_provider(my_cookie_store)](crate::ClientBuilder::cookie_provider)
+    /// is used; calling [cookie_store(true)](crate::ClientBuilder::cookie_store) _after_ [cookie_provider(my_cookie_store)](crate::ClientBuilder::cookie_provider) will result
+    /// in the provided `my_cookie_store` being **overridden** with a default implementation.
     ///
     /// # Optional
     ///
@@ -807,7 +811,10 @@ impl ClientBuilder {
     /// Cookies received in responses will be passed to this store, and
     /// additional requests will query this store for cookies.
     ///
-    /// By default, no cookie store is used.
+    /// By default, no cookie store is used. It is **not** necessary to also call
+    /// [cookie_store(true)](crate::ClientBuilder::cookie_store) if [cookie_provider(my_cookie_store)](crate::ClientBuilder::cookie_provider) is used; calling
+    /// [cookie_store(true)](crate::ClientBuilder::cookie_store) _after_ [cookie_provider(my_cookie_store)](crate::ClientBuilder::cookie_provider) will result
+    /// in the provided `my_cookie_store` being **overridden** with a default implementation.
     ///
     /// # Optional
     ///