Add doc-strs to `get_root()` + `maybe_open_portal()`

Brief descriptions for both fns in `._discovery` clarifying
what each delivers and under what conditions.

(this commit msg was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

tractor/_discovery.py

@@ -91,10 +91,13 @@ async def get_registry(
 
 
 @acm
-async def get_root(
+async def get_root(**kwargs) -> AsyncGenerator[Portal, None]:
-    **kwargs,
+    '''
-) -> AsyncGenerator[Portal, None]:
+    Deliver the current actor's "root process" actor (yes in actor
+    and proc tree terms) by delivering a `Portal` from the spawn-time
+    provided contact address.
 
+    '''
     # TODO: rename mailbox to `_root_maddr` when we finally
     # add and impl libp2p multi-addrs?
     addr = _runtime_vars['_root_mailbox']
@@ -193,6 +196,11 @@ async def maybe_open_portal(
     addr: UnwrappedAddress,
     name: str,
 ):
+    '''
+    Open a `Portal` to the actor serving @ `addr` or `None` if no
+    peer can be contacted or found.
+
+    '''
     async with query_actor(
         name=name,
         regaddr=addr,