Need better docs about Atomic Updates + Optimistic Concurrency

Atomic Update features are mentioned in passing in the Ref Guide, but not explained as well as on the wiki (and doesn't refer to it as "atomic updates" so it's hard to find)

https://cwiki.apache.org/confluence/display/solr/Uploading+Data+with+Index+Handlers#UploadingDatawithIndexHandlers-UpdatingOnlyPartofaDocument
https://wiki.apache.org/solr/Atomic_Updates

In particula, The Optimistic Concurrency options on atomic updatesare not really documented at all.

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(Initial issue description requested a "check" option that could be used on the uniqueKey field when doing atomic updates that would cause hte updated to fail if the uniqueKey specified did not exist – but this type of feature is not needed since we already support the equivilent optimisitic concurrency garuntees )