Merge branch 'docs-validation-service-clarification' into 'master'

Copy edit pipeline validation service doc

See merge request gitlab-org/gitlab!61462

doc/administration/external_pipeline_validation.md

@@ -5,31 +5,28 @@ info: To determine the technical writer assigned to the Stage/Group associated w
 type: reference, howto
 ---
 
-# External Pipeline Validation
+# External pipeline validation
 
 You can use an external service to validate a pipeline before it's created.
 
 WARNING:
 This is an experimental feature and subject to change without notice.
 
-## Usage
-
 GitLab sends a POST request to the external service URL with the pipeline
-data as payload. GitLab then invalidates the pipeline based on the response
-code. If there's an error or the request times out, the pipeline is not
-invalidated.
-
-Response codes:
+data as payload. The response code from the external service determines if GitLab
+should accept or reject the pipeline. If the response is:
 
-- `200`: Accepted
-- `4XX`: Rejected
-- All other codes: accepted and logged
+- `200`, the pipeline is accepted.
+- `4XX`, the pipeline is rejected.
+- Other codes, the pipeline is accepted and logged.
 
-### Service Result
+If there's an error or the request times out, the pipeline is accepted.
 
-Pipelines rejected by the external validation service aren't created or visible in pipeline lists, in either the GitLab user interface or API. Creating an unaccepted pipeline when using the GitLab user interface displays an error message that states: `Pipeline cannot be run. External validation failed`
+Pipelines rejected by the external validation service aren't created, and don't
+appear in pipeline lists in the GitLab UI or API. If you create a pipeline in the
+UI that is rejected, `Pipeline cannot be run. External validation failed` is displayed.
 
-## Configuration
+## Configure external pipeline validation
 
 To configure external pipeline validation, add the
 [`EXTERNAL_VALIDATION_SERVICE_URL` environment variable](environment_variables.md)
@@ -39,7 +36,7 @@ By default, requests to the external service time out after five seconds. To ove
 the default, set the `EXTERNAL_VALIDATION_SERVICE_TIMEOUT` environment variable to the
 required number of seconds.
 
-## Payload Schema
+## Payload schema
 
 ```json
 {