Allow optional keys to be absent

text/0000-security-advisories.md

@@ -108,11 +108,10 @@ A correct invocation makes Cargo do the following:
   """
   ```

2. After `$EDITOR` exits, Cargo validates the content. This includes verifying
   that:
2. After `$EDITOR` exits, Cargo validates the content. Some rules:

   - the file is valid TOML
   - none of the table keys have ben removed
   - Optional keys may be either `false` or absent.
   - the `description` contains not only whitespace. More text than a paragraph
     should be allowed, but not wished.
   - `package` exists on Crates.io and the versions specified in `versions` exist