docs/library/espnow: Clarify usage of the "rate" configuration key.

This commit adds a clarification for the ESPNow module's documentation
regarding its "config" method.

The original documentation for that method could be interpreted as having
all its configuration keys being able to be queried, but the "rate"
configuration key is actually write-only due to ESP-IDF's lack of a way to
retrieve that bit of information from the radio's configuration.  The
documentation changes highlight the fact that said configuration key is
actually write-only.

Signed-off-by: Alessandro Gatti <a.gatti@frob.it>

docs/library/espnow.rst

@@ -164,11 +164,13 @@ Configuration
         wait forever. The timeout can also be provided as arg to
         `recv()`/`irecv()`/`recvinto()`.
 
-        *rate*: (ESP32 only, IDF>=4.3.0 only) Set the transmission speed for
+        *rate*: (ESP32 only) Set the transmission speed for
         ESPNow packets. Must be set to a number from the allowed numeric values
         in `enum wifi_phy_rate_t
-        <https://docs.espressif.com/projects/esp-idf/en/v4.4.1/esp32/
-        api-reference/network/esp_wifi.html#_CPPv415wifi_phy_rate_t>`_.
+        <https://docs.espressif.com/projects/esp-idf/en/v5.2.3/esp32/
+        api-reference/network/esp_wifi.html#_CPPv415wifi_phy_rate_t>`_. This
+        parameter is actually *write-only* due to ESP-IDF not providing any
+        means for querying the radio interface's rate parameter.
 
     .. data:: Returns: