Refactor BaseBuilder.php and fix xxxBatch() methods docs

[kenjis]

Description

• refactor variable name in system/Database/BaseBuilder.php
• fix insertBatch() / updateBatch() / setUpdateBatch() user guide
• add tests for insertBatch() / updateBatch() with array of objects param

insertBatch() updateBatch() the first param

The User Guide says "You can either pass an array or an object to the function." But the the first param type is ?array in the code.

In fact, you can pass an array of arrays or objects.

Checklist:

• [x] Securely signed commits
• [ ] Component(s) with PHPdocs
• [ ] Unit testing, with >80% coverage
• [x] User guide updated
• [x] Conforms to style guide

[kenjis]

Related #5149

[sclubricants]

Can this be closed now?

[kenjis]

Is there test case for array of objects now?

[sclubricants]

Is there test case for array of objects now?

UpsertTest::testUpsertAndUpsertBatchWithObject()

[kenjis]

Thank you!