os: document behavior of IsNotExist, IsExist, etc. for nil errors #31065
Description
The IsNotExist, IsExist, IsPermission and IsTimeout functions in package os all return false when passed a nil error, but the documentation isn't too clear about that. I presume the behavior is intentional, since there are tests for it (except for IsTimeout?). Being able to rely on this property would be nice, as it allows one to write code like the following, to treat such errors specially:
if os.IsNotExist(err) {
// ...
} else if err != nil {
// ...
}
I think the root of the issue is that the term "the error" in these functions' docs is ambiguous - it could mean an error value (which can be nil) or an error condition (which is represented by a non-nil error value).