As we know, in command line arguments braces mean <required argument> and [optional argument], and additionally | in [1|2|3] and <1|2|3> means a selection of mutually exclusive arguments, which true for most of Minecraft's commands. Now, look at the situation bellow:

As seen on the screenshot, the teleport command syntax suggestion makes no sense. It means, literally:
Optional argument: either Facing or required Rotation
Basically, there a pair of optional arguments, one of which is mandatory to be chosen, which wrong both because the second option doesn't even exist and the suggestion of a "mandatory option" makes no logical sense.

Judging by the actual command syntax, the correct suggestion should be
[facing <entity|x y z>] and typing entity should lead to a suggestion of entities or selectors like with the target argument (which, by the way, doesn't happen here unlike all other entity arguments in the game)

When the facing argument is entered, the command syntax continues to defy logic:

It then gives an optional mandatory argument, which is actually not optional. The same appears to show up for most of the game's commands - optional arguments appear to either not have any braces, or wrapped in both  [optional] and <required>