Update docs (find(:foo) was removed)

find(:foo) was removed in version 2.0.0 (I don't know the reason but probably it was considered a bad practice). So I think reference to this way should be removed from docs too.
Merge pull request #924 from abotalov/patch-2

Update docs (find(:foo) was removed)
Remove info about thin from rdoc

Capybara doesn't use thin from version 2.0.2
Merge pull request #925 from abotalov/patch-2

Remove info about thin from rdoc
Document actual within_frame support