Improve the RemoteNode.load_configuration() docs

[erlend-aasland]

Spell out how it works and link to PdoBase.read()

Moreover, add return annotation to make sure that's included in the
rendered docs.

[erlend-aasland]

Shamelessly based off of #405 (comment), in a slightly more concise wording.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.65%. Comparing base (5734f37) to head (3da568c).
Report is 1 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #477      +/-   ##
==========================================
+ Coverage   67.55%   67.65%   +0.09%     
==========================================
  Files          26       26              
  Lines        3101     3104       +3     
  Branches      521      521              
==========================================
+ Hits         2095     2100       +5     
+ Misses        864      862       -2     
  Partials      142      142              

Files	Coverage Δ
canopen/node/remote.py	55.12% <100.00%> (ø)

... and 1 file with indirect coverage changes

[erlend-aasland]

Thanks for the review!