docs: update multi-user info

[ltalirz]

I've had a report that the previous text gave the impression that AiiDA
stores SSH keys in the database.
Revising the section, adding some extra info on the concept of users in
AiiDA, and mentioning explicitly that no sensitive data is stored in the
db/file repo.

[codecov]

Codecov Report

Merging #5338 (5600ab5) into develop (63e3146) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           develop    #5338      +/-   ##
===========================================
- Coverage    82.14%   82.13%   -0.00%     
===========================================
  Files          533      533              
  Lines        38485    38485              
===========================================
- Hits         31608    31606       -2     
- Misses        6877     6879       +2     

Flag	Coverage Δ
django	77.21% <0.00%> (-<0.01%)	⬇️
sqlalchemy	76.51% <0.00%> (-<0.01%)	⬇️

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Impacted Files	Coverage Δ
aiida/orm/nodes/data/structure.py	83.86% <0.00%> (ø)
aiida/engine/daemon/client.py	75.38% <0.00%> (-1.00%)	⬇️

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[sphuber]

Excellent, thanks a lot @ltalirz