[7.2.x] add CI and BUILD_NUMBER env var in docs (#10750)

Co-authored-by: bitzge <[email protected]>

Author: github-actions[bot]

changelog/10690.doc.rst

@@ -0,0 +1 @@
+Added `CI` and `BUILD_NUMBER` environment variables to the documentation.

doc/en/reference/reference.rst

@@ -1047,6 +1047,14 @@ Environment Variables
 
 Environment variables that can be used to change pytest's behavior.
 
+.. envvar:: CI
+
+When set (regardless of value), pytest acknowledges that is running in a CI process. Alterative to ``BUILD_NUMBER`` variable.
+
+.. envvar:: BUILD_NUMBER
+
+When set (regardless of value), pytest acknowledges that is running in a CI process. Alterative to CI variable.
+
 .. envvar:: PYTEST_ADDOPTS
 
 This contains a command-line (parsed by the py:mod:`shlex` module) that will be **prepended** to the command line given