expvarom: Add EOF marker, and minor documentation updates

This patch adds the EOF marker as required by the new specification, and
also adds some links to it in the comments, as reference.

internal/expvarom/expvarom_test.go

@@ -73,6 +73,7 @@ testMXI{key="key3"} 7
 # Generated by expvarom
 # EXPERIMENTAL - Format is not fully standard yet
 # Ignored variables: ["cmdline" "memstats" "testS"]
+# EOF
 `
 
 func TestHandler(t *testing.T) {