Consistent precondition formatting

Our "Preconditions:" blocks are very useful to determine the input invariants,
but they are bit inconsistent throughout the codebase, which makes them harder
to read (particularly cases with 5+ conditions in a single paragraph).

I've reformatted all of the cases to fit in simple rules:

1. Cases with a single condition are placed on a single line.
2. Cases with multiple conditions are placed in a bulleted list.

This format has been added to the style guide.

I've also mentioned "Postconditions:", though those are much less frequently
used, and all uses already match this style.

PiperOrigin-RevId: 327687465

pkg/unet/unet.go

@@ -522,7 +522,7 @@ func (s *ServerSocket) Listen() error {
 // This is always blocking.
 //
 // Preconditions:
-//  * ServerSocket is listening (Listen called).
+// * ServerSocket is listening (Listen called).
 func (s *ServerSocket) Accept() (*Socket, error) {
 	fd, ok := s.socket.enterFD()
 	if !ok {