Clarification on the use of certificate chains

[PE1NUT]

This pull request adds a little more detail to the documentation on the use of TLS certificates. As it took me a while to figure out why gitea would work with the server certificate, but not the full chain (which was ordered incorrectly), I think that a bit of clarification may help others in the future. Feel free to reject if this is considered canonical knowledge when working with X509 certificates.

This would close issue #12972.

[codecov-commenter]

Codecov Report

Merging #12986 into master will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #12986      +/-   ##
==========================================
- Coverage   42.59%   42.59%   -0.01%     
==========================================
  Files         671      671              
  Lines       73625    73625              
==========================================
- Hits        31363    31358       -5     
- Misses      37180    37184       +4     
- Partials     5082     5083       +1     

Impacted Files	Coverage Δ
modules/queue/workerpool.go	58.77% <0.00%> (-1.23%)	⬇️
services/pull/pull.go	40.78% <0.00%> (-0.50%)	⬇️

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