Documentation: i2c: add fault code for not supporting 10 bit addresses

Document the specific fault code when 10 bit addresses cannot be
supported. It is used for years, only the documentation slipped through
the cracks.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>

Author: Wolfram Sang

Documentation/i2c/fault-codes.rst

@@ -39,6 +39,10 @@ Also, codes returned by adapter probe methods follow rules which are
 specific to their host bus (such as PCI, or the platform bus).
 
 
+EAFNOSUPPORT
+	Returned by I2C adapters not supporting 10 bit addresses when
+	they are requested to use such an address.
+
 EAGAIN
 	Returned by I2C adapters when they lose arbitration in master
 	transmit mode:  some other master was transmitting different