docs: improve session

[kenjis]

Description
Small improvements or fixes.

If we agree, I would like to remve all sample code that uses $_SESSION directly.

Checklist:

• Securely signed commits
• [] Component(s) with PHPDoc blocks, only if necessary or adds value
• [] Unit testing, with >80% coverage
• User guide updated
• [] Conforms to style guide

[MGatner]

I think the UG used "userdata" throughout to indicate "non-flash data stored by the framework", because some actual $_SESSION items are not included:

$_exclude = array_merge(['__ci_vars'], $this->getFlashKeys(), $this->getTempKeys());

I'm fine with making this a little more opaque but we should note somewhere what is not included.

Otherwise: I am in favor of encouraging use of the Session handler over the global variable.

[kenjis]

There are some notes (important) to describe the items that the get() method returns
like the following:

The get() method WILL return flashdata or tempdata items when retrieving a single item by key. It will not return flashdata or tempdata when grabbing all data from the session, however.

And I added a note for the word "userdata".

[MGatner]

Sorry, I missed those other comments. Thank you for updating. LGTM!