Allow `Stop` passthrough from `PldRx.recv_msg_w_pld()`

Since we need to allow it (at the least) inside
`drain_until_final_msg()` for handling stream-phase termination races
where we don't want to have to handle a raised error from something like
`Context.result()`. Expose the passthrough option via
a `passthrough_non_pld_msgs: bool` kwarg.

Add comprehensive comment to `current_pldrx()`.

tractor/msg/_ops.py

@@ -355,6 +355,9 @@ class PldRx(Struct):
         ipc: Context|MsgStream,
         expect_msg: MsgType,
 
+        # NOTE: generally speaking only for handling `Stop`-msgs that
+        # arrive during a call to `drain_to_final_msg()` above!
+        passthrough_non_pld_msgs: bool = True,
         **kwargs,
 
     ) -> tuple[MsgType, PayloadT]:
@@ -365,6 +368,11 @@ class PldRx(Struct):
         '''
         msg: MsgType = await ipc._rx_chan.receive()
 
+        if passthrough_non_pld_msgs:
+            match msg:
+                case Stop():
+                    return msg, None
+
         # TODO: is there some way we can inject the decoded
         # payload into an existing output buffer for the original
         # msg instance?
@@ -389,15 +397,30 @@ _ctxvar_PldRx: ContextVar[PldRx] = ContextVar(
 
 def current_pldrx() -> PldRx:
     '''
-    Return the current `trio.Task.context`'s msg-payload
-    receiver, the post IPC but pre-app code `MsgType.pld`
-    filter.
+    Return the current `trio.Task.context`'s msg-payload-receiver.
+
+    A payload receiver is the IPC-msg processing sub-sys which
+    filters inter-actor-task communicated payload data, i.e. the
+    `PayloadMsg.pld: PayloadT` field value, AFTER it's container
+    shuttlle msg (eg. `Started`/`Yield`/`Return) has been delivered
+    up from `tractor`'s transport layer but BEFORE the data is
+    yielded to application code, normally via an IPC primitive API
+    like, for ex., `pld_data: PayloadT = MsgStream.receive()`.
 
     Modification of the current payload spec via `limit_plds()`
-    allows an application to contextually filter typed IPC msg
-    content delivered via wire transport.
+    allows a `tractor` application to contextually filter IPC
+    payload content with a type specification as supported by
+    the interchange backend.
+
+    - for `msgspec` see <PUTLINKHERE>.
+
+    NOTE that the `PldRx` itself is a per-`Context` global sub-system
+    that normally does not change other then the applied pld-spec
+    for the current `trio.Task`.
 
     '''
+    # ctx: context = current_ipc_ctx()
+    # return ctx._pld_rx
     return _ctxvar_PldRx.get()