Add documentation clarifying about request syntax

[swetashre]

Fixes #2086

[codecov-commenter]

Codecov Report

Merging #2142 into develop will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #2142      +/-   ##
===========================================
+ Coverage    93.09%   93.11%   +0.01%     
===========================================
  Files           60       60              
  Lines        11114    11146      +32     
===========================================
+ Hits         10347    10379      +32     
  Misses         767      767              

Impacted Files	Coverage Δ
botocore/docs/method.py	100.00% <100.00%> (ø)
botocore/discovery.py	100.00% <0.00%> (ø)
botocore/exceptions.py	99.46% <0.00%> (+<0.01%)	⬆️
botocore/model.py	98.24% <0.00%> (+0.03%)	⬆️
botocore/client.py	98.62% <0.00%> (+0.03%)	⬆️

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