docs: update funcs documentation with go 1.19 doc standards

[morremeyer]

go 1.19 introduced more features for documentation comments.

swagger switching to use tabs to indent (swaggo/swag#1386) broke the formatting for the backend since it did not have any actual go docs beyond the swagger comments yet.

With this PR, go docs are added, resolving this issue.

[codecov]

Codecov Report

Base: 87.74% // Head: 87.74% // No change to project coverage 👍

Coverage data is based on head (476a1c2) compared to base (11fe175).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #499   +/-   ##
=======================================
  Coverage   87.74%   87.74%           
=======================================
  Files          24       24           
  Lines        2937     2937           
=======================================
  Hits         2577     2577           
  Misses        282      282           
  Partials       78       78           

Impacted Files	Coverage Δ
pkg/controllers/account.go	96.10% <ø> (ø)
pkg/controllers/allocation.go	98.07% <ø> (ø)
pkg/controllers/budget.go	66.93% <ø> (ø)
pkg/controllers/category.go	94.70% <ø> (ø)
pkg/controllers/cleanup.go	24.32% <ø> (ø)
pkg/controllers/envelope.go	95.00% <ø> (ø)
pkg/controllers/import.go	77.27% <ø> (ø)
pkg/controllers/month.go	81.52% <ø> (ø)
pkg/controllers/month_config.go	92.70% <ø> (ø)
pkg/controllers/transaction.go	97.42% <ø> (ø)
... and 1 more

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