Update Script_Header.adoc

Author: lihaoyi

website/docs/modules/ROOT/partials/Script_Header.adoc

@@ -1 +1,10 @@
-This page documents the usage of single-file {language} modules, or "scripts".
+This page documents the usage of single-file {language} modules, or "scripts". These are 
+{language} modules with a single source file, with build configuration included
+in a header comment block at the top of the file.
+
+{language} scripts can be standalone or part of a larger Mill project. They can be run via
+`./mill Foo.{language-ext}` from the command line, or have other tasks on them executed
+via `./mill Foo.{language-ext}:compile` or `./mill Foo.{language-ext}:assembly`. 
+
+Apart from being limited to a single source file, single-file script modules otherwise
+support all the same tasks that normal ``{language}Module``s do.