Auto-merge button text in Verify-related docs

What does this MR do?

This MR updates the docs to represent the changes listed in the associated MR.

String before	String after
Merge when pipeline succeeds	Set to auto-merge
Add to merge train when pipeline succeeds	Set to auto-merge
Start merge train	Merge
Start merge train when pipeline succeeds	Set to auto-merge
Remove from merge train	Cancel auto-merge

Details

While the button name on merge requests has changed, these settings explain the behavior behind the scenes.

What we called "merge when pipeline succeeds" is controlled by this setting (In the project's Settings > Merge requests):

And merge trains are configured here (Also in the project's Settings > Merge requests):

Related issues

Related to: !115436 (merged)

Related to: technical-writing#782