Clarify maximum length of an invoice description

[tnull]

We previously stated in the docs that the invoice description can be at most 1023 bytes long, which is wrong. According to BOLT 11 it's at most 1023*5 bits (639 bytes) long.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.73%. Comparing base (dd5d297) to head (fa7ecc5).
Report is 8 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3185      +/-   ##
==========================================
- Coverage   89.82%   89.73%   -0.10%     
==========================================
  Files         121      121              
  Lines      100404   100834     +430     
  Branches   100404   100834     +430     
==========================================
+ Hits        90190    90480     +290     
- Misses       7550     7691     +141     
+ Partials     2664     2663       -1     

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[TheBlueMatt]

Lol, that's not a clarification, that's a correction. The old thing was very clearly wrong.

[TheBlueMatt]

Docs-only and the code trivially matches the new docs.