Adds more documentation about http modules

[caesar-ralf]

To improve the situation due to issue #362

[codecov]

Codecov Report

Merging #363 (5d1394e) into 1.x (70a761b) will decrease coverage by 0.09%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##                1.x     #363      +/-   ##
============================================
- Coverage     72.24%   72.15%   -0.10%     
  Complexity      676      676              
============================================
  Files           137      137              
  Lines          2868     2869       +1     
  Branches        169      169              
============================================
- Hits           2072     2070       -2     
- Misses          758      761       +3     
  Partials         38       38              

Impacted Files	Coverage Δ
...m/spotify/apollo/http/client/HttpMetricModule.java	0.00% <ø> (ø)
...otify/apollo/http/client/OkHttpClientProvider.java	100.00% <ø> (ø)
...m/spotify/apollo/http/client/HttpClientModule.java	100.00% <100.00%> (ø)
...main/java/com/spotify/apollo/core/ServiceImpl.java	90.37% <0.00%> (-1.04%)	⬇️

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