net: document the meaning of "Temporary" and "Timeout"

[jmhodges]

The docs for net have a lot of Temporary and Timeout error methods in them, but the package level docs don't contain a discussion of what those errors mean. The Temporary methods are mentioned in blog post about error handling but don't actually describe their meaning in detail.

And then for the errors types specifically, if an error type have both Temporary and Timeout, it'd be nice to document if both can be true at once and what that means semantically. (This originally cropped up when trying to explain if and when an error in a lower-level network protocol would or would not be counted against a rate limit in a higher-level one.)

[seankhliao]

Related #45729

[neild]

Temporary is deprecated in CL 340261. Please don't use it; I can pretty much guarantee that it doesn't do what you want, whatever that may be. See #45729 for an exhaustive list of places where the standard library returns an error with a Temporary method returning true.

I think Timeout is reasonably self-explanatory, and the more subtle details are better documented in specific functions returning errors. For example, Conn.SetDeadline states that the error returned when a deadline is exceeded will have a Timeout method that returns true.