Document build.commands shell environment

Author: LecrisUT

docs/user/config-file/v2.rst

@@ -487,6 +487,11 @@ The ``$READTHEDOCS_OUTPUT/html`` directory will be uploaded and hosted by Read t
 
    ``build.os`` and ``build.tools`` are also required when using ``build.commands``.
 
+.. note::
+
+   All items in the ``build.commands`` array are executed in a clean shell environment, i.e. environment changes do not
+   persist and the working directory always start from the git repo.
+
 :Type: ``list``
 :Required: ``false``
 :Default: ``[]``