[packages] Explain pre-script isn't actually a shell script

src/modules/packages/packages.conf

@@ -87,9 +87,25 @@ update_system: false
 #     pre-script: touch /tmp/installing-vi
 #     post-script: rm -f /tmp/installing-vi
 #
-# The pre- and post-scripts are optional, but you cannot leave both out: using
+# The pre- and post-scripts are optional, but you cannot leave both out
-# "package: vi" with neither script option will trick Calamares into
+# if you do use the *package* key: using "package: vi" with neither script
-# trying to install a package named "package: vi", which is unlikely to work.
+# option will trick Calamares into trying to install a package named
+# "package: vi", which is unlikely to work.
+#
+# The pre- and post-scripts are **not** executed by a shell unless you
+# explicitly invoke `/bin/sh` in them. The command-lines are passed
+# to exec(), which does not understand shell syntax. In other words:
+#
+#     pre-script: ls | wc -l
+#
+# Will fail, because `|` is passed as a command-line argument to ls,
+# as are `wc`, and `-l`. No shell pipeline is set up, and ls is likely
+# to complain. Invoke the shell explicitly:
+#
+#     pre-script: /bin/sh -c \"ls | wc -l\"
+#
+# The above note on shell-expansion applies to versions up-to-and-including
+# Calamares 3.2.12, but will change in future.
 #
 # Any package name may be localized; this is used to install localization
 # packages for software based on the selected system locale. By including