docs: update docs around email reports

[eschutho]

SUMMARY

I noticed while debugging with this flag that you have to run superset init in order for the menu/pages to appear. Some people have apparently gotten bit by something similar in the past, and it takes a while to figure out if you don't know what you're looking for, so I added it to the docs.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TEST PLAN

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Requires DB Migration.
• Confirm DB Migration upgrade and downgrade tested.
• Introduces new feature or API
• Removes existing feature or API

[codecov-io]

Codecov Report

Merging #11783 (b91a668) into master (cdd01f4) will decrease coverage by 8.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #11783      +/-   ##
==========================================
- Coverage   63.14%   55.12%   -8.03%     
==========================================
  Files         897      411     -486     
  Lines       43460    14521   -28939     
  Branches     4188     3725     -463     
==========================================
- Hits        27443     8004   -19439     
+ Misses      15837     6517    -9320     
+ Partials      180        0     -180     

Flag	Coverage Δ
cypress	55.12% <ø> (?)
javascript	?
python	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...uperset-frontend/src/dashboard/util/dnd-reorder.js	0.00% <0.00%> (-100.00%)	⬇️
...rset-frontend/src/dashboard/util/getEmptyLayout.js	0.00% <0.00%> (-100.00%)	⬇️
...dashboard/components/resizable/ResizableHandle.jsx	0.00% <0.00%> (-100.00%)	⬇️
.../src/dashboard/util/getFilterScopeFromNodesTree.js	0.00% <0.00%> (-93.48%)	⬇️
...src/dashboard/components/gridComponents/Header.jsx	10.52% <0.00%> (-86.85%)	⬇️
...rc/dashboard/components/gridComponents/Divider.jsx	13.33% <0.00%> (-86.67%)	⬇️
...ontend/src/dashboard/util/getDashboardFilterKey.ts	14.28% <0.00%> (-85.72%)	⬇️
...set-frontend/src/views/CRUD/welcome/EmptyState.tsx	5.71% <0.00%> (-82.10%)	⬇️
...et-frontend/src/components/ListView/ActionsBar.tsx	11.11% <0.00%> (-81.20%)	⬇️
...et-frontend/src/components/Menu/LanguagePicker.tsx	0.00% <0.00%> (-80.00%)	⬇️
... and 809 more

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