Describe colon ecosystems (#2036)

Add a description of the : sub ecosystems and how we export all sub
ecosystems in one grouped ecosystem. Feel free to reword because I'm not
sure I described this very well.

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Co-authored-by: Oliver Chang <[email protected]>

Author: another-rex

docs/data.md

@@ -104,6 +104,8 @@ E.g. for PyPI vulnerabilities:
 gsutil cp gs://osv-vulnerabilities/PyPI/all.zip .
 ```
 
+Some ecosystems contain a `:` separator in the name (e.g. `Alpine:v3.17`). For these ecosystems, the data dump will always contain an ecosystem directory without the `:.*` suffix (e.g. `Alpine`). This will contain all the advisories of the ecosystem with the same prefix (e.g. All `Alpine:.*`).
+
 A list of all current ecosystems is available at
 [`gs://osv-vulnerabilities/ecosystems.txt`](https://osv-vulnerabilities.storage.googleapis.com/ecosystems.txt)