Change encryption option name to "encrypt_tabs"

[hluk]

Also adds a note about encryption to the documentation.

[codecov]

Codecov Report

❌ Patch coverage is 95.65217% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.32%. Comparing base (c3fda41) to head (dc84d6a).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/gui/mainwindow.cpp	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3367      +/-   ##
==========================================
- Coverage   80.35%   80.32%   -0.04%     
==========================================
  Files         238      238              
  Lines       29864    29864              
==========================================
- Hits        23998    23987      -11     
- Misses       5866     5877      +11     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.