hook_post_update_NAME() docs do not explain batching/$sandbox parameter

Problem/Motivation

The docs for the $sandbox parameter of hook_post_update_NAME() say:

 * @param array $sandbox
 *   Stores information for batch updates. See above for more information. 

...But there's no "above". This is the only time the sandbox size or batching in general are mentioned in the entire docblock.

It was obviously copied from the hook_update_N() docs, which do include an "above" paragraph explaining batching.

Proposed resolution

Copy the relevant docs to the docblock.

Remaining tasks

NR.

User interface changes

N/A

API changes

N/A

Data model changes

N/A

Release notes snippet

N/A (docs addition).