docs/esp32: Correct quickref for ESP32 hardware SPI with non-default IO.
@@ -128,6 +128,8 @@ with timer ID of -1::
The period is in milliseconds.
+.. _Pins_and_GPIO:
+
Pins and GPIO
-------------
@@ -274,8 +276,13 @@ class::
Hardware SPI bus
----------------
-There are two hardware SPI channels that allow faster (up to 80Mhz)
-transmission rates, but are only supported on a subset of pins.
+There are two hardware SPI channels that allow faster transmission
+rates (up to 80Mhz). These may be used on any IO pins that support the
+required direction and are otherwise unused (see :ref:`Pins_and_GPIO`)
+but if they are not configured to their default pins then they need to
+pass through an extra layer of GPIO multiplexing, which can impact
+their reliability at high speeds. Hardware SPI channels are limited
+to 40MHz when used on pins other than the default ones listed below.
===== =========== ============
\ HSPI (id=1) VSPI (id=2)
