Improving documentation.

[tomaszdrozdz]

@ashleysommer @ahopkins
As promised and discussed in #1942 - tiny documentation improvements.
Deliberately not touching middlewares, listeners because there is another discussion pending around them.

Would it be good idea to keep this branch open so anyone anytime can improve documentation ?

[tomaszdrozdz]

And how about "introspection" section (#1949) ?
Should we have a new file for it in docs/sanic/introspection.rst ??

[codecov]

Codecov Report

Merging #1951 into master will increase coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1951      +/-   ##
==========================================
+ Coverage   92.38%   92.47%   +0.09%     
==========================================
  Files          29       29              
  Lines        3242     3242              
  Branches      571      571              
==========================================
+ Hits         2995     2998       +3     
+ Misses        169      167       -2     
+ Partials       78       77       -1     

Impacted Files	Coverage Δ
sanic/exceptions.py	100.00% <ø> (ø)
sanic/server.py	81.61% <0.00%> (+0.56%)	⬆️

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