Doc 9912 4.0 migration / transactions docs

[osfameron]

Note that some of this work was already committed to release/4.0 as WIP, so you may prefer to look at the full files rather than diffs.

@avsej some specific things to note for transactions-example.php:

• I'm not sure which exception maps to TransactionCommitAmbiguousError, have I got that the right way round?
• scope parameter doesn't work? (have commented out some examples that call this out)
• mutationCount metric doesn’t work?
• DurabilityLevel thing we discussed last week
• have I got the wrong/old method name for the transaction configuration? (I may have committed this before Sergey updated, and need to rebuild my PHP SDL 4.0)
• Renamed $attempt to $ctx (I liked $attempt for PHP and wanted to keep, but this makes it consistent with other examples, and @maria-robobug / @programmatix common partials work)

Changed some behaviour from Node.js version (for example pass the Cluster and Collection explicitly rather than awaiting, as I couldn't see if that was important?)

[osfameron]

Merging despite outstanding requests from @programmatix in order to get built on staging -- will create separate PR to address these issues!