Add set-security-context to feature flags docs

Prior to this commit, the set-security-context feature flag was documented
in the feature flags configmap and installation instructions, but not listed under
config options in "customizing the pipelines controller behavior".
This commit adds this flag to the list of feature flags in this doc.

docs/additional-configs.md

@@ -242,6 +242,17 @@ enables [beta features](#beta-features). When using v1 APIs, setting this field
 allows only stable features, and setting it to "beta" allows only beta features.
 Set this field to "alpha" to allow [alpha features](#alpha-features) to be used.
 
+For example:
+
+```yaml
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: feature-flags
+data:
+  enable-api-fields: "alpha" # Allow alpha fields to be used in Tasks and Pipelines.
+```
+
 - `trusted-resources-verification-no-match-policy`: Setting this flag to `fail` will fail the taskrun/pipelinerun if no matching policies found. Setting to `warn` will skip verification and log a warning if no matching policies are found, but not fail the taskrun/pipelinerun. Setting to `ignore` will skip verification if no matching policies found.
 Defaults to "ignore".
 
@@ -253,16 +264,8 @@ Defaults to "ignore".
   source from where a remote Task/Pipeline definition was fetched. By default, this is set to `true`.
   To disable populating this field, set this flag to `"false"`.
 
-For example:
-
-```yaml
-apiVersion: v1
-kind: ConfigMap
-metadata:
-  name: feature-flags
-data:
-  enable-api-fields: "alpha" # Allow alpha fields to be used in Tasks and Pipelines.
-```
+- `set-security-context`: Set this flag to `true` to set a security context for containers injected by Tekton that will allow TaskRun pods
+to run in namespaces with `restricted` pod security admission. By default, this is set to `false`.
 
 ### Alpha Features