Document Sort Merge Join algorithm

[comphead]

Is your feature request related to a problem or challenge?

I faced that it is challenging to understand fully how Sort Merge Join sort_merge_join.rs implemented and would be nice to get it documented the same way as HashJoin in hash_join.rs

Describe the solution you'd like

No response

Describe alternatives you've considered

No response

Additional context

No response

[comphead]

Found when fixing SMJ bugs https://github.com/apache/datafusion/pull/10304

[comphead]

cc @korowa you might be interested

[alamb]

Maybe @metesynnada can help remember who knows about this code too

[viirya]

I spent some time reading and understanding the code last time I worked with it. I may try to document it some once I get time to refresh it...