Explain prefix/root better in docs

mock/docs/site-defaults.cfg

@@ -657,8 +657,9 @@
 # Whether to use host's shadow-utils to provision users and groups in the
 # buildroot, which we normally want to do because host shadow-utils are
 # newer and more flexible than buildroot ones.However, there is an issue in shadow-utils
-# where even using the --root option, the host config will "leak" into the chroot. This is not an
-# issue if the configs are effectively the same between host and buildroot, but will cause
-# problems if, for example, the host is configured to use FreeIPA-provided subids.
+# where even using the --prefix (or, even --root if we did it that way) option, the host
+# config will "leak" into the chroot. This is not an issue if the configs are
+# effectively the same between host and buildroot, but will cause problems if, for
+# example, the host is configured to use FreeIPA-provided subids.
 # See https://github.com/shadow-maint/shadow/issues/897
 # config_opts["use_host_shadow_utils"] = True

mock/py/mockbuild/config.py

@@ -355,9 +355,10 @@ def setup_default_config_opts():
 
     config_opts["copy_host_users"] = []
 
-    # shadow-utils --root option does not play well with FreeIPA-provided
-    # subids. Using the shadow-utils inside the chroot works around this
-    # but this is a niche situation so it is not the default.
+    # shadow-utils --prefix and --root options do not play well with
+    # FreeIPA-provided subids. Using the shadow-utils inside the
+    # chroot works around this but this is a niche situation so it is
+    # not the default.
     # Upstream issue https://github.com/shadow-maint/shadow/issues/897
     config_opts["use_host_shadow_utils"] = True