fix(schema): document the use of special spans for implicit modules (#…

…4625)

* fix(schema): document the use of special spans for implicit modules

* fix(schema): clarify wording and use defines edge

kythe/docs/schema/schema.txt

@@ -2066,6 +2066,10 @@ package
 
 Brief description::
   A *package* defines a module containing declarations.
+Notes::
+  Languages in which a module is implicitly defined based on the file name
+  should emit a `defines` edge from a zero-width anchor at offset 0 in that
+  file to the corresponding package node.
 
 [kythe,Java,"Top-level declarations are children of package nodes."]
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