docs: Add comment to explain `recId` and `v` fields of `EthTxData`

[david-bakin-sl]

Description:

There's been confusion over the EthTxData fields recId and v. Add comments to the code to guide future readers. (This came up in looking at issues w.r.t. EIP-155.)

Related issue(s):

Fixes #13990

Checklist

• [x] Documented (Code comments, README, etc.)
• [N/A] Tested (unit, integration, etc.)

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Node: Unit Test Results

  1 562 files    1 562 suites   2h 34m 22s :stopwatch: 111 849 tests 111 790 :white_check_mark: 59 :zzz: 0 :x: 120 143 runs  120 084 :white_check_mark: 59 :zzz: 0 :x:

Results for commit 75680b23.

:recycle: This comment has been updated with latest results.

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Node: HAPI Test (Node Death Reconnect) Results

3 tests   3 :white_check_mark:  5m 50s :stopwatch: 3 suites  0 :zzz: 3 files    0 :x:

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Node: HAPI Test (Token) Results

 20 files   20 suites   6m 0s :stopwatch: 265 tests 265 :white_check_mark: 0 :zzz: 0 :x: 340 runs  340 :white_check_mark: 0 :zzz: 0 :x:

Results for commit 75680b23.

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Node: HAPI Test (Restart) Results

3 tests   3 :white_check_mark:  6m 5s :stopwatch: 3 suites  0 :zzz: 3 files    0 :x:

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Node: HAPI Test (Crypto) Results

 24 files   24 suites   11m 37s :stopwatch: 358 tests 358 :white_check_mark: 0 :zzz: 0 :x: 364 runs  364 :white_check_mark: 0 :zzz: 0 :x:

Results for commit 75680b23.

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Node: HAPI Test (Misc) Results

 51 files   51 suites   20m 47s :stopwatch: 357 tests 357 :white_check_mark: 0 :zzz: 0 :x: 375 runs  375 :white_check_mark: 0 :zzz: 0 :x:

Results for commit 75680b23.

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Node: HAPI Test (Smart Contract) Results

 72 files   72 suites   20m 48s :stopwatch: 610 tests 610 :white_check_mark: 0 :zzz: 0 :x: 662 runs  662 :white_check_mark: 0 :zzz: 0 :x:

Results for commit 75680b23.

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Node: HAPI Test (Time Consuming) Results

19 tests   19 :white_check_mark:  23m 39s :stopwatch:  4 suites   0 :zzz:  4 files     0 :x:

Results for commit 75680b23.