Improve API docs

[ldclakmal]

Purpose

$subject
Related to ballerina-platform/ballerina-library#584

Checklist

• Linked to an issue
• Updated the changelog
• Added tests

[codecov]

Codecov Report

Merging #202 (c768e27) into master (066daa4) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #202   +/-   ##
=========================================
  Coverage     89.05%   89.05%           
  Complexity       34       34           
=========================================
  Files             8        8           
  Lines           548      548           
  Branches        192      192           
=========================================
  Hits            488      488           
  Misses           52       52           
  Partials          8        8           

Impacted Files	Coverage Δ	Complexity Δ
oauth2-ballerina/client_oauth2_provider.bal	92.18% <ø> (ø)	0.00 <0.00> (ø)
oauth2-ballerina/listener_oauth2_provider.bal	90.15% <ø> (ø)	0.00 <0.00> (ø)
oauth2-ballerina/oauth2_commons.bal	100.00% <ø> (ø)	0.00 <0.00> (ø)

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