i2c: update i2c-dev.h warning in documentation

`Documentation/i2c/dev-interface` gives examples for accessing i2c from
userspace.

There's a note that warns developers about the two `i2c-dev.h` header
files which were shipped with the kernel and i2c-tools separately.

However, following i2c-tools commits suggest that the header files are now
identical (in functionality) and `i2c_*` helper functions are now defined
in a separate header called `i2c/smbus.h`, which is distributed with
i2c-tools:

commit 652619121974 ("Minimize differences with kernel flavor")
commit 93caf007f4cb ("Move SMBus helper functions to include/i2c/smbus.h")

Thus, I've converted the warning paragraph into a historical note and
updated the suggested header files.

Signed-off-by: Cengiz Can <cengizc@gmail.com>
Cc: Wolfram Sang <wsa@the-dreams.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/i2c/dev-interface

@@ -17,13 +17,16 @@ i2c-10, ...). All 256 minor device numbers are reserved for i2c.
 C example
 =========
 
-So let's say you want to access an i2c adapter from a C program. The
+So let's say you want to access an i2c adapter from a C program.
-first thing to do is "#include <linux/i2c-dev.h>". Please note that
+First, you need to include these two headers:
-there are two files named "i2c-dev.h" out there, one is distributed
+
-with the Linux kernel and is meant to be included from kernel
+  #include <linux/i2c-dev.h>
-driver code, the other one is distributed with i2c-tools and is
+  #include <i2c/smbus.h>
-meant to be included from user-space programs. You obviously want
+
-the second one here.
+(Please note that there are two files named "i2c-dev.h" out there. One is
+distributed with the Linux kernel and the other one is included in the
+source tree of i2c-tools. They used to be different in content but since 2012
+they're identical. You should use "linux/i2c-dev.h").
 
 Now, you have to decide which adapter you want to access. You should
 inspect /sys/class/i2c-dev/ or run "i2cdetect -l" to decide this.