docs: Add a description of the fast wakeup

docs/en/api-guides/bootloader.rst

@@ -55,6 +55,10 @@ After the GPIO input is deactivated and the device reboots, the normally configu
 
 :ref:`CONFIG_BOOTLOADER_HOLD_TIME_GPIO` - this is hold time of GPIO for reset/test mode (by default 5 seconds). The GPIO must be held low continuously for this period of time after reset before a factory reset or test partition boot (as applicable) is performed.
 
+Fast boot from Deep Sleep
+-------------------------
+The bootloader has the :ref:`CONFIG_BOOTLOADER_SKIP_VALIDATE_IN_DEEP_SLEEP` option which allows to reduce the wake-up time (useful to reduce consumption). This option is available when the :ref:`CONFIG_SECURE_BOOT_ENABLED` option is disabled. Reduction of time is achieved due to the lack of image verification. During the first boot, the bootloader stores the address of the application being launched in the RTC FAST memory. And during the awakening, this address is used for booting without any checks, thus fast loading is achieved.
+
 Customer bootloader
 ---------------------
 The current bootloader implementation allows the customer to override it. To do this, you must copy the folder `/esp-idf/components/bootloader` and then edit `/your_project/components/bootloader/subproject/main/bootloader_main.c`.

docs/en/api-guides/deep-sleep-stub.rst

@@ -88,4 +88,4 @@ For example, the equivalent example in ``rtc_wake_stub_counter.c``::
 
 The second way is a better option if you need to use strings, or write other more complex code.
 
-
+To reduce wake-up time use the `CONFIG_BOOTLOADER_SKIP_VALIDATE_IN_DEEP_SLEEP` Kconfig option, see more information in :doc:`Fast boot from Deep Sleep <bootloader>`.