Mention eboot use of rtc memory (#5439)

Author: mcspr

doc/libraries.rst

@@ -75,7 +75,7 @@ Some ESP-specific APIs related to deep sleep, RTC and flash memories are availab
 
 ``ESP.deepSleepInstant(microseconds, mode)`` works similarly to ``ESP.deepSleep`` but  sleeps instantly without waiting for WiFi to shutdown.
 
-``ESP.rtcUserMemoryWrite(offset, &data, sizeof(data))`` and ``ESP.rtcUserMemoryRead(offset, &data, sizeof(data))`` allow data to be stored in and retrieved from the RTC user memory of the chip respectively. Total size of RTC user memory is 512 bytes, so ``offset + sizeof(data)`` shouldn't exceed 512. Data should be 4-byte aligned. The stored data can be retained between deep sleep cycles. However, the data might be lost after power cycling the chip.
+``ESP.rtcUserMemoryWrite(offset, &data, sizeof(data))`` and ``ESP.rtcUserMemoryRead(offset, &data, sizeof(data))`` allow data to be stored in and retrieved from the RTC user memory of the chip respectively. ``offset`` is measured in blocks of 4 bytes and can range from 0 to 127 blocks (total size of RTC memory is 512 bytes). ``data`` should be 4-byte aligned. The stored data can be retained between deep sleep cycles, but might be lost after power cycling the chip. Data stored in the first 32 blocks will be lost after performing an OTA update, because they are used by the Core internals.
 
 ``ESP.restart()`` restarts the CPU.
 

doc/ota_updates/readme.rst

@@ -614,7 +614,9 @@ The Stream Interface is the base for all other update modes like OTA, http Serve
 Updater class
 -------------
 
-Updater is in the Core and deals with writing the firmware to the flash, checking its integrity and telling the bootloader to load the new firmware on the next boot.
+Updater is in the Core and deals with writing the firmware to the flash, checking its integrity and telling the bootloader (eboot) to load the new firmware on the next boot.
+
+**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>`__.
 
 Update process - memory view
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~