CODING_STLYE: Simplify explanation for use of 'extern'

CODING_STLYE: Simplify explanation for use of 'extern'

Current explanation of rules around usage of 'extern' are overly
verbose.  It is not necessary to state that functions should be declared
in header files, the compiler already enforces this.  These rules are
simple, they are better described with simple statements.  An example is
not necessary for such simple rules and serves only to make the document
longer.

Use two simple statements describing the rules that govern function
declaration and the usage of the 'extern' keyword.

Signed-off-by: Tobin C. Harding <me@tobin.cc>