extmod/modnetwork: Increase max hostname length to 32.

This changes from the previous limit of 15 characters.  Although DHCP and
mDNS allow for up to 63, ESP32 and ESP8266 only allow 32, so this seems
like a reasonable limit to enforce across all ports (and avoids wasting the
additional memory).

Also clarifies that `MICROPY_PY_NETWORK_HOSTNAME_MAX_LEN` does not include
the null terminator (which was unclear before).

This work was funded through GitHub Sponsors.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

docs/library/network.rst

@@ -188,6 +188,11 @@ The following are functions available in the network module.
     during connection. For this reason, you must set the hostname before
     activating/connecting your network interfaces.
 
+    The length of the hostname is limited to 32 characters.
+    :term:`MicroPython ports <MicroPython port>` may choose to set a lower
+    limit for memory reasons. If the given name does not fit, a `ValueError`
+    is raised.
+
     The default hostname is typically the name of the board.
 
 .. function:: phy_mode([mode])