Update description of --enable-symlinks

Update description of --enable-symlinks

As most of the symlinks in our setup are, in fact, independent
of the Ganeti version, they can be installed unconditionally. So
the --enable-symlinks configure option now only controls a small
number of symlinks that decide which version of Ganeti is used.
Update the documentation accordingly.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>