docs: how to approach borg speed issues, fixes #5371

[ThomasWaldmann]

also: give a speed example for unchanged files

also: explicitly mention chmod/chown changing ctime

also: reformat one paragraph

[ThomasWaldmann]

@fantasya-pbem can you review, please?

[codecov-commenter]

Codecov Report

Merging #5377 into 1.1-maint will increase coverage by 0.67%.
The diff coverage is n/a.

@@              Coverage Diff              @@
##           1.1-maint    #5377      +/-   ##
=============================================
+ Coverage      79.53%   80.20%   +0.67%     
=============================================
  Files             27       27              
  Lines          10396    10396              
  Branches        1829     1829              
=============================================
+ Hits            8268     8338      +70     
+ Misses          1612     1548      -64     
+ Partials         516      510       -6     

Impacted Files	Coverage Δ
src/borg/archiver.py	82.05% <ø> (ø)
src/borg/helpers.py	87.41% <0.00%> (+0.42%)	⬆️
src/borg/archive.py	83.13% <0.00%> (+0.74%)	⬆️
src/borg/platform/base.py	83.17% <0.00%> (+3.73%)	⬆️
src/borg/platform/__init__.py	89.47% <0.00%> (+15.78%)	⬆️
src/borg/xattr.py	82.21% <0.00%> (+22.59%)	⬆️

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[ThomasWaldmann]

merging. if changes are needed, they can be applied in a future PR also.