Mention OTA Update class blocks flash mode changes (#7321)

Add mention that OTA Update class will block changes to flash mode bits if the image is uncompressed, and link to the recent discussion on this.

Co-authored-by: Earle F. Philhower, III <[email protected]>

Author: pfeerick

doc/ota_updates/readme.rst

@@ -669,6 +669,8 @@ Updater is in the Core and deals with writing the firmware to the flash, checkin
 
 **Note:** The bootloader command will be stored into the first 128 bytes of user RTC memory, then it will be retrieved by eboot on boot. That means that user data present there will be lost `(per discussion in #5330) <https://github.com/esp8266/Arduino/pull/5330#issuecomment-437803456>`__.
 
+**Note:** For uncompressed firmware images, the Updater will change the flash mode bits if they differ from the flash mode the device is currently running at. This ensures that the flash mode is not changed to an incompatible mode when the device is in a remote or hard to access area. Compressed images are not modified, thus changing the flash mode in this instance could result in damage to the ESP8266 and/or flash memory chip or your device no longer be accessible via OTA, and requiring re-flashing via a serial connection `(per discussion in #7307) <https://github.com/esp8266/Arduino/issues/7307#issuecomment-631523053>`__.
+
 Update process - memory view
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~