added links to client config documentation

[tpltnt]

Hi there,

I added a few references I wish I could click on when reading configuration documentation (again).

Cheers,
tpltnt

[codecov-io]

Codecov Report

Merging #456 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #456      +/-   ##
==========================================
+ Coverage   90.07%   90.09%   +0.01%     
==========================================
  Files         144      144              
  Lines       27296    27296              
  Branches     3925     3925              
==========================================
+ Hits        24586    24591       +5     
+ Misses       1966     1963       -3     
+ Partials      744      742       -2

Impacted Files	Coverage Δ
src/allmydata/web/status.py	83.08% <0%> (+0.22%)	⬆️
src/allmydata/immutable/upload.py	95.53% <0%> (+0.27%)	⬆️

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[meejah]

Great, thanks!