docs(banker): Add TotalCoin reference

[leohhhn]

This PR adds a reference to the TotalCoin function in the native banker.

This PR should not be merged before the functionality is implemented. Reference code here.

Contributors' checklist...

• Added new tests, or not needed, or not feasible
• Provided an example (e.g. screenshot) to aid review or the PR is self-explanatory
• Updated the official documentation or not needed
• No breaking changes were made, or a BREAKING CHANGE: xxx message was included in the description
• Added references to related issues and PRs
• Provided any useful hints for running manual tests
• Added new benchmarks to generated graphs, if any. More info here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.12%. Comparing base (1f58ee4) to head (84e2951).
Report is 169 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2661   +/-   ##
=======================================
  Coverage   60.11%   60.12%           
=======================================
  Files         560      560           
  Lines       74911    74911           
=======================================
+ Hits        45036    45037    +1     
  Misses      26500    26500           
+ Partials     3375     3374    -1     

Flag	Coverage Δ
contribs/gnodev	61.40% <ø> (ø)
contribs/gnofaucet	14.46% <ø> (ø)
gno.land	64.18% <ø> (ø)
gnovm	64.13% <ø> (ø)
misc/genstd	80.54% <ø> (ø)
misc/logos	20.23% <ø> (+0.35%)	⬆️
tm2	62.05% <ø> (+0.08%)	⬆️

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[github-actions]

This PR is stale because it has been open 3 months with no activity. Remove stale label or comment or this will be closed in 3 months.