Document how to disable undercover code coverage

doc/development/pipelines.md

@@ -37,7 +37,7 @@ flowchart LR
     subgraph backend
     be["Backend code"]--tested with-->rspec
     end
-    
+
     be--generates-->fixtures["frontend fixtures"]
     fixtures--used in-->jest
 ```
@@ -252,6 +252,16 @@ In the event of an emergency, or false positive from this job, add the
 `pipeline:skip-undercoverage` label to the merge request to allow this job to
 fail.
 
+You can disable the `undercover` code coverage check by wrapping the desired block of code in `# :nocov:` lines:
+
+```ruby
+# :nocov:
+def some_method
+  # code coverage for this method will be skipped
+end
+# :nocov:
+```
+
 ## PostgreSQL versions testing
 
 Our test suite runs against PG12 as GitLab.com runs on PG12 and