Docs: explain that instances may also exist for weight-setting

settings.conf

@@ -26,11 +26,29 @@
 modules-search: [ local ]
 
 # Instances section. This section is optional, and it defines custom instances
-# for modules of any kind. An instance entry has an module name, an instance
-# name, and a configuration file name. The primary goal of this mechanism is
-# to allow loading multiple instances of the same module, with different
-# configuration. If you don't need this, the instances section can safely be
-# left empty.
+# for modules of any kind. An instance entry has these keys:
+# - *module* name, which matches the module name from the module descriptor
+#   (usually the name of the directory under `src/modules/`, but third-
+#   party modules may diverge.
+# - *id* (optional) an identifier to distinguish this instance from
+#   all the others. If none is given, the name of the module is used.
+#   Together, the module and id form an instance key (see below).
+# - *config* (optional) a filename for the configuration. If none is
+#   given, *module*`.conf` is used (e.g. `welcome.conf` for the welcome
+#   module)
+# - *weight* (optional) In the *exec* phase of the sequence, progress
+#   is reported as jobs are completed. The jobs from a single module
+#   together contribute the full weight of that module. The overall
+#   progress (0 .. 100%) is divided up according to the weight of each
+#   module. Give modules that take a lot of time to complete, a larger
+#   weight to keep the overall progress moving along steadily. This
+#   weight overrides a weight given in the module descriptor. If no weight
+#   is given, uses the value from the module descriptor, or 1 if there
+#   isn't one there either.
+#
+# The primary goal of this mechanism is to allow loading multiple instances
+# of the same module, with different configuration. If you don't need this,
+# the instances section can safely be left empty.
 #
 # Module name plus instance name makes an instance key, e.g.
 # "webview@owncloud", where "webview" is the module name (for the webview
@@ -43,10 +61,11 @@ modules-search: [ local ]
 # mentioning a module without a full instance key (e.g. "welcome")
 # means that implicit module.
 #
-# An instance must specify its configuration file (e.g. `webview-home.conf`).
+# An instance may specify its configuration file (e.g. `webview-home.conf`).
 # The implicit instances all have configuration files named `<module>.conf`.
 # This (implict) way matches the source examples, where the welcome
-# module contains an example `welcome.conf`.
+# module contains an example `welcome.conf`. Specify a *config* for
+# any module (also implicit instances) to change which file is used.
 #
 # For more information on running module instances, run Calamares in debug
 # mode and check the Modules page in the Debug information interface.