[packages] Document *localInstall* operation.

src/modules/packages/packages.conf

@@ -64,6 +64,8 @@ update_system: false
 # - localInstall: this is used to call the package manager
 #       to install a package from a path-to-a-package. This is
 #       useful if you have a static package archive on the install media.
+#       The *pacman* package manager is the only one to specially support
+#       this operation (all others treat this the same as *install*).
 # - remove, try_remove: will call the package manager to
 #       remove one or more packages. The remove target will
 #       abort the whole installation if package-removal fails,