Added new functions to documentation

doc/serenity_docs/serenity_api.txt

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+#title Serenity3D [RCBasic Doc]
+#header Serenity3D API
+
+This API is what allows developers to access objects created in the Serenity3D editor inside RCBasic.
+
+
+'-------SKY-------
+Function Serenity_GetSkyShape()
+
+Function Serenity_GetSkyBox() As Serenity_Sky_Box
+
+Function Serenity_GetSkyDome() As Serenity_Sky_Dome
+
+Function Serenity_StageHasSky()
+
+
+
+'-------STAGES-------
+Function Serenity_GetStageIndex(stage_name$)
+
+Function Serenity_GetStageName$(stage_index)
+
+Function Serenity_GetStageCount()
+
+Sub Serenity_ClearStage()
+
+Sub Serenity_LoadStage(stage_name$)
+
+Function Serenity_GetCurrentStageIndex()
+
+'-------GROUPS-------
+Function Serenity_GetGroupCount()
+
+Function Serenity_GetGroupIndex(group_name$)
+
+Function Serenity_GetGroupName$(group_index)