docs: i2c: rename sections so the overall picture is clearer

Some of the section names are not very clear. Reading those names in the
index.rst page does not help much in grasping what the content is supposed
to be.

Rename those sections to clarify their content, especially when reading
the index page.
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Acked-by: Peter Rosin <peda@axentia.se>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

Documentation/i2c/dev-interface.rst

-====================
-I2C Device Interface
-====================
+============================================
+Implementing I2C device drivers in userspace
+============================================
 
 Usually, I2C devices are controlled by a kernel driver. But it is also
 possible to access all devices on an adapter from userspace, through

Documentation/i2c/i2c-protocol.rst

-============
-I2C Protocol
-============
+================
+The I2C Protocol
+================
 
 This document describes the I2C protocol. Or will, when it is finished :-)
 

Documentation/i2c/i2c-topology.rst

-============
-I2C topology
-============
+================================
+I2C muxes and complex topologies
+================================
 
 There are a couple of reasons for building more complex I2C topologies
 than a straight-forward I2C bus with one adapter and one or more devices.

Documentation/i2c/old-module-parameters.rst

-=================================================
-I2C device driver binding control from user-space
-=================================================
+================================================================
+I2C device driver binding control from user-space in old kernels
+================================================================
 
 .. NOTE::
    Note: this section is only relevant if you are handling some old code

Documentation/i2c/smbus-protocol.rst

-======================
-SMBus Protocol Summary
-======================
+==================
+The SMBus Protocol
+==================
 
 The following is a summary of the SMBus protocol. It applies to
 all revisions of the protocol (1.0, 1.1, and 2.0).

Documentation/i2c/summary.rst

-=============
-I2C and SMBus
-=============
+=============================
+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

Documentation/i2c/writing-clients.rst

-===================
-Writing I2C Clients
-===================
+===============================
+Implementing I2C device drivers
+===============================
 
 This is a small guide for those who want to write kernel drivers for I2C
 or SMBus devices, using Linux as the protocol host/master (not slave).