man: describe "symlink" and "systemctl link" explicitly in UNIT FILE LOAD PATH

man: describe "symlink" and "systemctl link" explicitly in UNIT FILE LOAD PATH

There are sometimes users who put unit files in a location that is inaccessible
when systemd starts although they are not found and thus not started because
the corresponding mount units have not activated yet.

There is already a warning for such issue in man 8 systemctl:

     link PATH...
         ...<snip>...
         The file system where
         the linked unit files are located must be accessible
         when systemd is started (e.g. anything underneath /home
         or /var is not allowed, unless those directories are
         located on the root file system).

However, it looks that it's difficult to find the warning because introductory
users typically doesn't know systemctl link.

Although there is a description in UNIT FILE LOAD PATH pointing to systemctl
link, symlink is now not explicitly mentioned there and thus users doesn't
easily get aware of they should read it.

To deal with this, let's describe "symlink" and "systemctl link" more
explicitly in UNIT FILE LOAD PATH.