Add each_batch_range to developer documentation

Ref: #333258 (closed)

What does this MR do?

This adds a new section to the developer documentation to explain how to modify existing data with our new helper method.

Why are we doing this?

Following up !63519 (merged) where we introduced a new new migration helpers for data migration in batches, it was brought up that we need to add it to our documentation.

Conformity

• [-] I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• [-] I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• [-] I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• [-] This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.