Update BTT comments for USB/SD Composite (#16130)

platformio.ini

 # BigTree SKR Mini V1.1 / SKR mini E3 / SKR E3 DIP (STM32F103RCT6 ARM Cortex-M3)
 #
 #   STM32F103RC_bigtree ............. RCT6 with 256K
-#   STM32F103RC_bigtree_USB ......... RCT6 with 256K (USB)
+#   STM32F103RC_bigtree_USB ......... RCT6 with 256K (USB mass storage)
 #   STM32F103RC_bigtree_512K ........ RCT6 with 512K
-#   STM32F103RC_bigtree_512K_USB .... RCT6 with 512K (USB)
+#   STM32F103RC_bigtree_512K_USB .... RCT6 with 512K (USB mass storage)
 #
 
 [env:STM32F103RC_bigtree]
 
 #
 #   STM32F103RE_bigtree ............. RET6
-#   STM32F103RE_bigtree_USB ......... RET6 (USB)
+#   STM32F103RE_bigtree_USB ......... RET6 (USB mass storage)
 #
 [env:STM32F103RE_bigtree]
 platform          = ststm32