Move theme toggling description to toggle viselement (#2378)

Co-authored-by: Fred Lefévère-Laoide <90181748+FredLL-Avaiga@users.noreply.github.com>

taipy/gui/viselements.json

@@ -285,7 +285,8 @@
                 ],
                 "properties": [
                     {
-                        "name": "value"
+                        "name": "value",
+                        "doc": "Bound to the selection value.<br/>If this is used for theme toggling, it must be a Boolean value: True indicates that the current theme is set to \"dark\", while False indicates that the theme is set to \"light\"."
                     },
                     {
                         "name": "theme",
@@ -2149,7 +2150,7 @@
                         "name": "value",
                         "default_property": true,
                         "type": "dynamic(Any)",
-                        "doc": "Bound to the selection value.<br/>If this is used for theme toggling, it must be a Boolean value: True indicates that the current theme is set to \"dark\", while False indicates that the theme is set to \"light\"."
+                        "doc": "Bound to the selection value."
                     },
                     {
                         "name": "lov",