Clarify and correct the exception rendering docs.

Refs #2112
Undocument the session component.

Remove the session component from the docs as it is deprecated.

Refs #1985
[fr] follow en version in #2119
Merge pull request #2119 from cakephp/remove-session-component

Undocument the session component.
Merge pull request #2114 from cakephp/clarify-error-controller

Clarify and correct the exception rendering docs.
Correct wording.