Improve docs for all the main functions

[sreecodeslayer]

As of now, Hex docs are empty for the functions. Although docs and some of the example usage is already present in the README, it would be nice to have the same and more on the hexdocs.

[paulhenri]

Hi,
Is it still needed ? Right now we can see there is no real documentation on hexdox.pm but some is already in the code.
I'd like to help on this if it is still needed.

[szymon-jez]

Yes, please go ahead.

[paulhenri]

Will do :) I may need some time to figure out what the code is doing but i'm on it.

[szymon-jez]

Some things to start from:

Generating docs...
warning: documentation references function "ExPhoneNumber.parse/2" but it is hidden
  README.md

warning: documentation references file "LICENSE.md" but it does not exist
  README.md

View "html" docs at "doc/index.html"
warning: documentation references function "ExPhoneNumber.parse/2" but it is hidden
  README.md

warning: documentation references file "LICENSE.md" but it does not exist
  README.md

View "epub" docs at "doc/ExPhoneNumber.epub"