Improve `Channel` doc-strs + minor cleanups

Flesh out missing method doc-strings, improve log msg formatting and
assert -> `RuntimeError` for un-inited tpt layer.

Deats,
- add doc-string to `.send()` noting `TransportClosed` raise
  on comms failures.
- add doc-string to `.recv()`.
- expand `._aiter_msgs()` doc-string, line-len reflow.
- add doc-string to `.connected()`.
- convert `assert self._transport` -> `RuntimeError` raise
  in `._aiter_msgs()` for more explicit crashing.
- expand `_connect_chan()` doc-string, note it's lowlevel
  and suggest `.open_portal()` to user instead.
- factor out `src_exc_str` in `TransportClosed` log handler
  to avoid double-call
- use multiline style for `.connected()` return expr.

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

tractor/ipc/_chan.py

@@ -307,7 +307,12 @@ class Channel:
 
     ) -> None:
         '''
-        Send a coded msg-blob over the transport.
+        Send a coded msg-blob over the underlying IPC transport.
+
+        This fn raises `TransportClosed` on comms failures and is
+        normally handled by higher level runtime machinery for the
+        expected-graceful cases, normally ephemercal
+        (re/dis)connects.
 
         '''
         __tracebackhide__: bool = hide_tb
@@ -334,9 +339,10 @@ class Channel:
                     except KeyError:
                         raise err
                 case TransportClosed():
+                    src_exc_str: str = err.repr_src_exc()
                     log.transport(
-                        f'Transport stream closed due to\n'
-                        f'{err.repr_src_exc()}\n'
+                        f'Transport stream closed due to,\n'
+                        f'{src_exc_str}'
                     )
 
                 case _:
@@ -345,6 +351,11 @@ class Channel:
             raise
 
     async def recv(self) -> Any:
+        '''
+        Receive the latest (queued) msg-blob from the underlying IPC
+        transport.
+
+        '''
         assert self._transport
         return await self._transport.recv()
 
@@ -418,16 +429,18 @@ class Channel:
         self
     ) -> AsyncGenerator[Any, None]:
         '''
-        Yield `MsgType` IPC msgs decoded and deliverd from
-        an underlying `MsgTransport` protocol.
+        Yield `MsgType` IPC msgs decoded and deliverd from an
+        underlying `MsgTransport` protocol.
 
-        This is a streaming routine alo implemented as an async-gen
-        func (same a `MsgTransport._iter_pkts()`) gets allocated by
-        a `.__call__()` inside `.__init__()` where it is assigned to
-        the `._aiter_msgs` attr.
+        This is a streaming routine alo implemented as an
+        async-generator func (same a `MsgTransport._iter_pkts()`)
+        gets allocated by a `.__call__()` inside `.__init__()` where
+        it is assigned to the `._aiter_msgs` attr.
 
         '''
-        assert self._transport
+        if not self._transport:
+            raise RuntimeError('No IPC transport initialized!?')
+
         while True:
             try:
                 async for msg in self._transport:
@@ -462,7 +475,15 @@ class Channel:
             #     continue
 
     def connected(self) -> bool:
-        return self._transport.connected() if self._transport else False
+        '''
+        Predicate whether underlying IPC tpt is connected.
+
+        '''
+        return (
+            self._transport.connected()
+            if self._transport
+            else False
+        )
 
     async def _do_handshake(
         self,
@@ -493,8 +514,11 @@ async def _connect_chan(
     addr: UnwrappedAddress
 ) -> typing.AsyncGenerator[Channel, None]:
     '''
-    Create and connect a channel with disconnect on context manager
-    teardown.
+    Create and connect a `Channel` to the provided `addr`, disconnect
+    it on cm exit.
+
+    NOTE, this is a lowlevel, normally internal-only iface. You
+    should likely use `.open_portal()` instead.
 
     '''
     chan = await Channel.from_addr(addr)