Documentation/CodingStyle: improve text layout

Try to make coding style documentation look a bit more readable and
consistent with the following:

 - indent every code example in C to first tab-stop;
 - surround every code example with empty lines, both top and bottom;
 - remove empty lines where text looked way too spare;
 - do not indent examples in elisp and kconfig;
 - do not do any non-whitespace changes.

Signed-off-by: Pavel Kretov <firegurafiku@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>