extcon: Correct description to improve the readability

The extcon files explains the detailed operation for functions and
what is meaning of extcon structure. There are different explanation
even if the same argument.

So, it modifies the description for both functions and structures
in order to improve the readability and guide the role of functions
more well.

Also, this patch fixes the mismatching license info as a GPL v2
and removes the inactive author information.

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>