Polish docs and hlint suggestions

Hi,

I noticed some small documentation issues and used the opportunity to also apply `hlint` suggestions.

Detailed list of changes:
- remove redundant sentence for `Aeson.decode`
- remove superfluous `the`
- apply HLint suggestions in Aeson and Aeson.Internal
- remove redundant sentence for `Aeson.decodeL`

[![24pullrequests](http://24pullrequests.com/assets/logo-625222452ffc0d57272decb6f851a7fe.png)](http://24pullrequests.com)

Best,
Markus