symlink: docs need an update

[kolyshkin]

moby/sys/symlink documentation (as seen at e.g. https://pkg.go.dev/github.com/moby/sys/symlink) needs a few updates:

1. Package docs say the package "implements EvalSymlinksInScope ..." while in fact it implements FollowSymlinkInScope.

2. Doc for FollowSymlinksInScope says it "is a wrapper around evalSymlinksInScope" which is not an adequate description as a user knows nothing about evalSymlinksInScope.

[kolyshkin]

Also, this is unrelated, but from the first glance it seems that golang's filepath.EvalSymlinks now support \\?\-like paths on Windows, which makes moby/sys/symlink.EvalSymlink obsoleted