specs: clarify Q3 extended data

[rootulp]

Context

#1442 (comment)

Proposal

Confirm that it doesn't matter if Q3 is extended from Q1 or Q2. Then add a note to

celestia-app/specs/src/specs/data_structures.md

Lines 387 to 391 in c09843d

	Where `A -> B` indicates that `B` is computed using [erasure coding](#reed-solomon-erasure-coding) from `A`:
	- `Q0 -> Q1` for each row in `Q0` and `Q1`
	- `Q0 -> Q2` for each column in `Q0` and `Q2`
	- `Q2 -> Q3` for each row in `Q2` and `Q3`

[staheri14]

We may need to also update the link added as part of this PR #1461 if the extension logic is explained in a different section of the data structure spec.

[rootulp]

Verified that Q3 is identical if it is extended vertically from Q1 or horizontally from Q2. See this commit and this test which continues to pass.

[staheri14]

Verified that Q3 is identical if it is extended vertically from Q1 or horizontally from Q2. See this commit and this test which continues to pass.

Great! Do you think it would make sense to add a comment to the code indicating that either direction i.e., Q1->Q3 or Q2->Q3 would have the same result (in addition to the specs).

[rootulp]

to add a comment to the code

I don't think it would hurt. Will add a comment to the code in rsmt2d.