docs: i2c: summary: update speed mode description

Fastest I2C mode is 5 MHz. Update the docs and reword the paragraph
slightly.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Easwar Hariharan <eahariha@linux.microsoft.com>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>

Author: Wolfram Sang

Documentation/i2c/summary.rst

@@ -3,8 +3,8 @@ Introduction to I2C and SMBus
 =============================
 
 I²C (pronounce: I squared C and written I2C in the kernel documentation) is
-a protocol developed by Philips. It is a slow two-wire protocol (variable
-speed, up to 400 kHz), with a high speed extension (3.4 MHz). It provides
+a protocol developed by Philips. It is a two-wire protocol with variable
+speed (typically up to 400 kHz, high speed modes up to 5 MHz). It provides
 an inexpensive bus for connecting many types of devices with infrequent or
 low bandwidth communications needs. I2C is widely used with embedded
 systems. Some systems use variants that don't meet branding requirements,