[libcalamares] Document how TranslatedString context works

- Support re-using class-specific tr() calls in a standard way
- Document this in the netinstall.conf which uses it

src/libcalamares/locale/TranslatableConfiguration.h

@@ -39,6 +39,17 @@ class DLLEXPORT TranslatedString
 {
 public:
     /** @brief Get all the translations connected to @p key
+     *
+     * Gets map[key] as the "untranslated" form, and then all the
+     * keys of the form <key>[lang] are taken as the translation
+     * for <lang> of the untranslated form.
+     *
+     * If @p context is not a nullptr, then that is taken as an
+     * indication to **also** use the regular QObject::tr() translation
+     * mechanism for these strings. It is recommended to pass in
+     * metaObject()->className() as context (from a QObject based class)
+     * to give the TranslatedString the same context as other calls
+     * to tr() within that class.
      */
     TranslatedString( const QVariantMap& map, const QString& key, const char* context = nullptr );
     /** @brief Not-actually-translated string.

src/modules/netinstall/netinstall.conf

@@ -30,8 +30,14 @@ required: false
 # If no *sidebar* values are provided, defaults to "Package selection"
 # and existing translations. If no *title* values are provided, no string
 # is displayed.
+#
+# The following strings are already known to Calamares and can be
+# listed here in *untranslated* form (e.g. as value of *sidebar*)
+# without bothering with the translations: they are picked up from
+# the regular translation framework:
+#   - "Package selection"
 label:
     sidebar: "Package selection"
-    sidebar[nl]: "Pakketkeuze"
+    # sidebar[nl]: "Pakketkeuze"
-    title: "Office Package"
+    # title: "Office Package"
-    title[nl]: "Kantoorsoftware"
+    # title[nl]: "Kantoorsoftware"