Include doc comments about the env vars.

Co-authored-by: Pete Gonzalez <4673363+octogonz@users.noreply.github.com>

Author: iclanton

build-tests/heft-sass-test/config/heft.json

@@ -16,6 +16,10 @@
             "pluginName": "set-environment-variables-plugin",
             "options": {
               "environmentVariablesToSet": {
+                // Suppress the "Browserslist: caniuse-lite is outdated" warning. Although the warning is
+                // potentially useful, the check is performed in a way that is nondeterministic and can cause
+                // Rush pipelines to fail. Moreover, the outdated version is often irrelevant and/or nontrivial
+                // to upgrade. See this thread for details: https://github.com/microsoft/rushstack/issues/2981
                 "BROWSERSLIST_IGNORE_OLD_DATA": "1"
               }
             }

rigs/heft-web-rig/profiles/app/config/heft.json

@@ -28,6 +28,10 @@
             "pluginName": "set-environment-variables-plugin",
             "options": {
               "environmentVariablesToSet": {
+                // Suppress the "Browserslist: caniuse-lite is outdated" warning. Although the warning is
+                // potentially useful, the check is performed in a way that is nondeterministic and can cause
+                // Rush pipelines to fail. Moreover, the outdated version is often irrelevant and/or nontrivial
+                // to upgrade. See this thread for details: https://github.com/microsoft/rushstack/issues/2981
                 "BROWSERSLIST_IGNORE_OLD_DATA": "1"
               }
             }

rigs/heft-web-rig/profiles/library/config/heft.json

@@ -28,6 +28,10 @@
             "pluginName": "set-environment-variables-plugin",
             "options": {
               "environmentVariablesToSet": {
+                // Suppress the "Browserslist: caniuse-lite is outdated" warning. Although the warning is
+                // potentially useful, the check is performed in a way that is nondeterministic and can cause
+                // Rush pipelines to fail. Moreover, the outdated version is often irrelevant and/or nontrivial
+                // to upgrade. See this thread for details: https://github.com/microsoft/rushstack/issues/2981
                 "BROWSERSLIST_IGNORE_OLD_DATA": "1"
               }
             }