singhularity: add comment to defaultToSuid

Add comment above "defaultToSuid = true" to explain it
and to keep it multi-line after formatting.

pkgs/applications/virtualization/singularity/packages.nix

···
       extraMeta.homepage = "https://sylabs.io/";
     })
     {
+      # Sylabs SingularityCE builders defaults to set the SUID flag
+      # on UNIX-like platforms,
+      # and only have --without-suid but not --with-suid.
       defaultToSuid = true;
     };