docs: Explained how the request URL is generated from url_base, `pa…

…th` and the sync context (#2530)

singer_sdk/streams/rest.py

@@ -77,7 +77,17 @@ class RESTStream(Stream, t.Generic[_TToken], metaclass=abc.ABCMeta):  # noqa: PL
     @property
     @abc.abstractmethod
     def url_base(self) -> str:
-        """Return the base url, e.g. ``https://api.mysite.com/v3/``."""
+        """The base request URL, e.g. ``https://api.mysite.com/v3/``.
+
+        Request URLs are generated by combining `url_base` and `path`, and expanding any
+        context variables in the path.
+
+        For example, if ``url_base`` is ``https://api.mysite.com/v3/`` and ``path`` is
+        ``users/{user_id}/orders``, then if the stream has a context of
+        ``{"user_id": 123}`` generated by its parent stream with
+        :meth:`~singer_sdk.Stream.generate_child_contexts`, the full URL will be
+        ``https://api.mysite.com/v3/users/123/orders``.
+        """
 
     def __init__(
         self,