fix: invoice item descriptions for missing document types

[shadrak98]

Item descriptions were only set for Site document type and set as Prepaid credits implicitly in the print format for all others which would make the invoice look something like this

[codecov]

Codecov Report

Attention: Patch coverage is 35.71429% with 9 lines in your changes missing coverage. Please review.

Project coverage is 38.88%. Comparing base (b5dd9c0) to head (76a3b52).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
press/press/doctype/invoice/invoice.py	35.71%	9 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2380   +/-   ##
=======================================
  Coverage   38.88%   38.88%           
=======================================
  Files         381      381           
  Lines       29396    29406   +10     
=======================================
+ Hits        11430    11435    +5     
- Misses      17966    17971    +5     

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