Warn in platformio.ini about RCT6 512K (#19312)

platformio.ini

 #   STM32F103RC_btt_512K ........ RCT6 with 512K
 #   STM32F103RC_btt_512K_USB .... RCT6 with 512K (USB mass storage)
 #
+# WARNING! If you have an SKR Mini v1.1 or an SKR Mini E3 1.0 / 1.2 / 2.0 / DIP
+# and experience a printer freeze, re-flash Marlin using the regular (non-512K)
+# build option. 256K chips may be re-branded 512K chips, but this means the
+# upper 256K is sketchy, and failure is very likely.
+#
 
 [env:STM32F103RC_btt]
 platform          = ${common_stm32f1.platform}