VDiff: Comment the VDiffCreate proto msg and set reasonable server side defaults

[mattlord]

Description

This is some internal cleanup that I wanted to do after helping some people use the VtctldServer VDiff RPCs directly. It's mostly comments, but it also enforces a couple of additional server side defaults in a sensible way (and which matches the official vtlctld client flag defaults).

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.15%. Comparing base (d651a42) to head (7470cf5).
Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #17026   +/-   ##
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  Coverage   67.15%   67.15%           
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  Files        1571     1571           
  Lines      251836   251840    +4     
=======================================
+ Hits       169110   169113    +3     
- Misses      82726    82727    +1     

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