better documentation of Git-sourced fields when parsing

The fact we use the Git history to deduce the *published* and *modified* date fields is
non-obvious and can confuse users, because parsing our advisory content will fail if
they are divorced from the Git repo. We should improve the documentation about this.

*original summary retained below*

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## Summary
- remove an advisory
- toml parsing fails
## Expected behavior
- toml parsing works on some bag of advisories, there shouldn't have to be any constraints on them
## Cause of the bug

Start by searching the repository documentation for the parsing behavior and references to the published and modified fields. Update the relevant documentation to explain that these dates are derived from Git history and describe the repository constraint that affects parsing; consider the work done when users can understand why detached advisory content may fail.

Written by the indexing model from the issue text.