docs: add --pkg-relationships

Signed-off-by: knqyf263 <[email protected]>

docs/docs/scanner/vulnerability.md

@@ -202,7 +202,8 @@ Currently, specifying a username and password is not supported.
 This section describes vulnerability-specific configuration.
 Other common options are documented [here](../configuration/index.md).
 
-### Enabling a subset of package types
+### Enabling a Subset of Package Types
+
 It's possible to only enable certain package types if you prefer.
 You can do so by passing the `--pkg-types` option.
 This flag takes a comma-separated list of package types.
@@ -268,6 +269,45 @@ Total: 7 (UNKNOWN: 0, LOW: 1, MEDIUM: 1, HIGH: 3, CRITICAL: 2)
 
 </details>
 
+!!! info
+    This flag filters the packages themselves, so it also affects the `--list-all-pkgs` option and SBOM generation.
+
+### Filtering by Package Relationships
+
+
+Trivy supports filtering vulnerabilities based on the relationship of packages within a project.
+This is achieved through the `--pkg-relationships` flag.
+This feature allows you to focus on vulnerabilities in specific types of dependencies, such as only those in direct dependencies.
+
+In Trivy, there are four types of package relationships:
+
+1. `root`: The root package being scanned
+2. `direct`: Direct dependencies of the root package
+3. `indirect`: Transitive dependencies
+4. `unknown`: Packages whose relationship cannot be determined
+
+The available relationships may vary depending on the ecosystem.
+To see which relationships are supported for a particular project, you can use the JSON output format and check the `Relationship` field:
+
+```
+$ trivy repo -f json --list-all-pkgs /path/to/project
+```
+
+To scan only the root package and its direct dependencies, you can use the flag as follows:
+
+```
+$ trivy repo --pkg-relationships root,direct /path/to/project
+```
+
+By default, all relationships are included in the scan.
+
+!!! info
+    This flag filters the packages themselves, so it also affects the `--list-all-pkgs` option and SBOM generation.
+
+!!! warning
+    As it may not provide a complete package list, it can lead to incomplete dependency trees when using `--dependency-tree` or generate incomplete SBOMs.
+
+
 [^1]: https://github.com/GoogleContainerTools/distroless
 
 [nvd-CVE-2023-0464]: https://nvd.nist.gov/vuln/detail/CVE-2023-0464