Add expanded documentation to AdapterInterface::update

[navarr]

This adds a long description to AdapterInterface::update detailing how the $where parameter works

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-cicd2]

All committers have signed the CLA.

[dmanners]

Hi @navarr thank you for this PR. I will process this now for you.

[dmanners]

Hi @navarr I am having a few issues with the branch. Is this branch up to date with the latest 2.3-develop branch?

[navarr]

@dmanners Yes. This branch is based off commit f5debf0 which is the latest in the 2.3-develop branch.

[magento-engcom-team]

Hi @navarr. Thank you for your contribution.
Changes from your Pull Request will be available with the upcoming 2.3.0 release.