This is a follow-up to #29875, which is closed.

Problems:

1. The fix was only applied to path.Join docs, but filepath.Join has the same problem and should be fixed too.
2. The new wording drops the sentence "all empty strings are ignored", leaving no clue that path.Join("", "foo") results in "foo" rather than "/foo".

New:

Join joins the argument's path elements into a single path, separating them with slashes. The result is Cleaned. However, if the argument list is empty or all its elements are empty, Join returns an empty string.

Old:

Join joins any number of path elements into a single path, adding a separating slash if necessary. The result is Cleaned; in particular, all empty strings are ignored.

I think it should be clear that Join can return the empty string, but it should be also clear that empty strings at the beginning don't make the path absolute.

While I'm at it, I'm also confused by path.Dir:

Dir returns all but the last element of path, typically the path's directory. After dropping the final element using Split, the path is Cleaned and trailing slashes are removed. If the path is empty, Dir returns ".". If the path consists entirely of slashes followed by non-slash bytes, Dir returns a single slash. In any other case, the returned path does not end in a slash.

It should probably read "If the path consists entirely of slashes possibly followed by non-slash bytes" or "If the path consists entirely of slashes followed by zero or more non-slash bytes", otherwise the last sentence ("In any other case ...") is false.