chore: update documentation for frontend feature flags

[etr2460]

SUMMARY

This was out of date, updating

TESTING INSTRUCTIONS

CI

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

to: @m-ajay @graceguo-supercat @ktmud

[codecov]

Codecov Report

Merging #15093 (005d6a5) into master (f8b270d) will increase coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #15093      +/-   ##
==========================================
+ Coverage   77.53%   77.61%   +0.08%     
==========================================
  Files         967      967              
  Lines       49740    49740              
  Branches     6351     6351              
==========================================
+ Hits        38565    38606      +41     
+ Misses      10973    10932      -41     
  Partials      202      202              

Flag	Coverage Δ
hive	81.44% <ø> (ø)
javascript	72.44% <ø> (ø)
mysql	81.70% <ø> (ø)
postgres	81.73% <ø> (ø)
presto	81.43% <ø> (?)
python	82.25% <ø> (+0.15%)	⬆️
sqlite	81.35% <ø> (ø)

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

Impacted Files	Coverage Δ
superset/models/core.py	90.02% <0.00%> (+0.26%)	⬆️
superset/connectors/sqla/models.py	89.81% <0.00%> (+1.42%)	⬆️
superset/db_engine_specs/presto.py	90.31% <0.00%> (+5.89%)	⬆️

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