Improve API documentation with query usages

[niveathika]

Purpose

All API doc except the batchExecute is updated with sample usage. batch Execute will need a code snippet.

Partially fixes ballerina-platform/ballerina-library#2524

Examples

N/A

Checklist

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

[codecov]

Codecov Report

Merging #379 (85d5958) into master (ac6187d) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #379   +/-   ##
=========================================
  Coverage     90.67%   90.67%           
  Complexity       40       40           
=========================================
  Files            13       13           
  Lines           268      268           
  Branches         45       45           
=========================================
  Hits            243      243           
  Misses           10       10           
  Partials         15       15           

Impacted Files	Coverage Δ
ballerina/client.bal	96.29% <ø> (ø)

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