docs: remove usage of deprecated tokio_core in docs

[SteveLauC]

What does this PR do

Update the documentation of RpcSystem

1. tokio_core has been deprecated, so replace tokio_core::reactor::handle::spawn() with tokio::task::spawn_local().
2. Explicitly document it that RpcSystem is not Send (so one needs spawn_local() rather than spawn()).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 51.64%. Comparing base (b85e041) to head (ab342b3).
Report is 14 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #492      +/-   ##
==========================================
- Coverage   51.84%   51.64%   -0.20%     
==========================================
  Files          69       69              
  Lines       34127    33735     -392     
==========================================
- Hits        17692    17422     -270     
+ Misses      16435    16313     -122     

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[dwrensha]

Thanks!