Add docs for semgrep vulnerability severities

doc/user/application_security/vulnerabilities/severities.md

@@ -50,6 +50,7 @@ the following tables:
 | [`pmd-apex`](https://gitlab.com/gitlab-org/security-products/analyzers/pmd-apex)                         | **{check-circle}** Yes   | Integer                    | `1`, `2`, `3`, `4`, `5`            |
 | [`kubesec`](https://gitlab.com/gitlab-org/security-products/analyzers/kubesec)                           | **{check-circle}** Yes   | String                     | `CriticalSeverity`, `InfoSeverity` |
 | [`secrets`](https://gitlab.com/gitlab-org/security-products/analyzers/secrets)                           | **{check-circle}** Yes   | N/A                        | Hardcodes all severity levels to `Critical` |
+| [`semgrep`](https://gitlab.com/gitlab-org/security-products/analyzers/semgrep)                           | **{check-circle}** Yes   | String                     | `error`, `warning`, `note`, `none` |
 
 ## Dependency Scanning