Documentation: explain module-paths better

- how "local" is interpreted
 - recommendations for other paths

settings.conf

@@ -7,8 +7,20 @@
 # path to a directory or the keyword "local".
 #
 # "local" means:
-#   - modules in $LIBDIR/calamares/moduleswith
+#   - modules in $LIBDIR/calamares/modules, with
 #   - settings in SHARE/calamares/modules or /etc/calamares/modules.
+# In debug-mode (e.g. calamares -d) "local" also adds some paths
+# that make sense from inside the build-directory, so that you
+# can build-and-run with the latest modules immediately.
+#
+# Strings other than "local" are taken as paths and interpreted
+# relative to wherever Calamares is started. It is therefore **strongly**
+# recommended to use only absolute paths here. This is mostly useful
+# if your distro has forks of standard Calamares modules, but also
+# uses some form of upstream packaging which might overwrite those
+# forked modules -- then you can keep modules somewhere outside of
+# the "regular" module tree.
+#
 #
 # YAML: list of strings.
 modules-search: [ local ]