docs(transaction encoding): sapling fields

[oxarbitrage]

Motivation

We want to make sure all the fields in the transaction tables of the protocol are documented in Zebra at serialization and deserialization.

This pull request deals with the sapling fields of transactions.

Closes #3422

Solution

Document sapling transaction fields.

Review

Anyone can review.

Reviewer Checklist

• All fields considered "sapling" are documented.

Follow Up Work

Orchard fields: #3424

[codecov]

Codecov Report

Merging #3501 (622d63a) into main (499ae89) will increase coverage by 0.20%.
The diff coverage is 74.75%.

@@            Coverage Diff             @@
##             main    #3501      +/-   ##
==========================================
+ Coverage   78.34%   78.54%   +0.20%     
==========================================
  Files         267      273       +6     
  Lines       31526    31902     +376     
==========================================
+ Hits        24698    25059     +361     
- Misses       6828     6843      +15     

[teor2345]

Looks good!

If you're looking to cut down on the work for future PRs, we can just mention each field the first time it is used in a Rust file.