Add FlashComponent and FlashHelper docs
Add namespace directives, add missing underscore
Remove flash stuff from session docs, use Flash classes in examples
Added information about new plugin structure and clarifying how to use the autoloader option
Updating more paths
Merge pull request #1434 from cakephp/3.0-plugin-changes

Added information about new plugin structure and clarifying how to use t...
Merge pull request #1430 from bcrowe/3.0-flash

3.0 - Add FlashComponent and FlashHelper docs
Update migration guide for FlashComponent/FlashHelper.

Remove docs for non-existing SessionHelper methods.
Fix syntax error.

Closes cakephp/cakephp#3753
Move note location in flash helper for consistency with the component