psbt: clarify PSBT, PSBTInput, PSBTOutput unserialization flows #32419

1. 
   rkrux commented at 11:41 am on May 5, 2025: contributor

   The unserialization flows of the PSBT types work based on few underlying assumptions of functions from serialize.h & stream.h that takes some to understand when read the first time.

   Add few comments that highlight these assumptions hopefully making it easier to grasp. Also, mention key/value format types as per BIP 174.

2. psbt: clarify PSBT, PSBTInput, PSBTOutput unserialization flows

   The unserialization flows of the PSBT types work based on few underlying
   assumptions of functions from `serialize.h` & `stream.h` that takes some
   to understand when read the first time.
   
   Add few comments that highlight these assumptions hopefully making it easier
   to grasp. Also, mention key/value format types as per BIP 174.

   d31158d364
3. 
   DrahtBot commented at 11:41 am on May 5, 2025: contributor

   The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

   Code Coverage & Benchmarks

   For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/32419.

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   Type	Reviewers
   Concept ACK	theStack, w0xlt

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4. DrahtBot added the label PSBT on May 5, 2025
5. 
   theStack commented at 4:55 pm on May 6, 2025: contributor

   Concept ACK

   Makes sense to me to provide at least a brief description of how the key data is structured and deserialized, since this is not intuitive at all (as I also noticed in the course of reviewing #31247 a while ago).

6. 
   w0xlt commented at 6:33 pm on May 7, 2025: contributor

   Concept ACK

   Agreed #32419 (comment)

Contributors
rkrux DrahtBot theStack w0xlt

Labels
PSBT