Update motivation.rst

Proper grammar in header
Merge pull request #164 from quasipickle/patch-10

Update motivation.rst
Advisory about storing data in cookies without encryption
Advisory about storing data in cookies without encryption (2)
Replace 1&1 owned "mywebsite.com" with neutral "example.com" domain.

example.com is registered by IANA for usage in documentation.
Merge pull request #165 from BilgeXA/patch-1

Replace 1&1 owned "mywebsite.com" with neutral "example.com" domain.
Merge remote-tracking branch 'original/master'
Безопасность сессий, правка ссылки на сайт примеров