Update Atomic Distributed Transaction Design

[harshit-gangal]

Description

This PR updated the Atomic Distributed Transaction design document.

Related Issue(s)

• RFC: Atomic Distributed Transactions #16245

Checklist

• 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 67.40%. Comparing base (0fa7e58) to head (6034ead).
Report is 74 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17005      +/-   ##
==========================================
- Coverage   69.34%   67.40%   -1.94%     
==========================================
  Files        1571     1570       -1     
  Lines      204180   252892   +48712     
==========================================
+ Hits       141582   170457   +28875     
- Misses      62598    82435   +19837     

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[systay]

Throughout the document, there are varying uses of terms like “vttablet,” “VTTablet,” “vtgate,” and “VTGate.” It would be better to standardize these terms for consistency. Use “VTTablet” and “VTGate” uniformly.
Also - use lowercase consistently when referring to commands and flags (e.g., transaction_mode=twopc).