Docs for Display and LowerHex improved, also support "1" version

[dr-orlovsky]

Found when was fixing RGB-WG/rgb#239 that we can improve here. Since this MUST not affect any existing code, I am just merging that in. CC @zoedberg @nicbus

[codecov]

Codecov Report

Attention: Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 42.6%. Comparing base (c3d2b8f) to head (98be887).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
psbt/src/data.rs	86.7%	2 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##           master     #41   +/-   ##
======================================
  Coverage    42.5%   42.6%           
======================================
  Files          24      24           
  Lines        4232    4238    +6     
======================================
+ Hits         1800    1804    +4     
- Misses       2432    2434    +2     

Flag	Coverage Δ
rust	42.6% <86.7%> (+<0.1%)	⬆️

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