docs: correct what make:migration --session reads from Config\Session

[paulbalandan]

Description

Discovered while doing #10545 .

The sessions guide says make:migration --session "will take the $savePath and $matchIP settings into account". That was true when the command was added in 094f675 (2017), but fc9add0 (2018) dropped the $savePath lookup in favor of a plain ci_sessions default, and the sentence was never updated. Today the command reads only $matchIP. The table name and database group come from --table and --dbgroup.

The paragraph now states that, shows both options, and notes that their values should match $savePath and $DBGroup.

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