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 #186 (c7fdf8b) into master (bc269d5) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #186   +/-   ##
=========================================
  Coverage     87.43%   87.43%           
  Complexity       74       74           
=========================================
  Files            15       15           
  Lines           374      374           
  Branches         53       53           
=========================================
  Hits            327      327           
  Misses           37       37           
  Partials         10       10           

Impacted Files	Coverage Δ	Complexity Δ
crypto-ballerina/encrypt_decrypt.bal	100.00% <ø> (ø)	0.00 <0.00> (ø)
crypto-ballerina/hash.bal	100.00% <ø> (ø)	0.00 <0.00> (ø)
crypto-ballerina/private_public_key.bal	100.00% <ø> (ø)	0.00 <0.00> (ø)

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