Clarify documentation of event_loop fixture

[seifertm]

• Removed contradictory information on whether the loop from the event_loop fixture will be available as the global event loop
• Replaced example of custom event loop with an example of redefining the scope of the event_loop fixture
• Added recommendation to use a custom event loop policy rather than redefining the event_loop fixture

Closes #288

[codecov-commenter]

Codecov Report

Merging #375 (08952ee) into master (f13c85f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #375   +/-   ##
=======================================
  Coverage   93.09%   93.09%           
=======================================
  Files           3        3           
  Lines         275      275           
  Branches       41       41           
=======================================
  Hits          256      256           
  Misses         12       12           
  Partials        7        7           

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