Write doc about X-FHIR-Query as part of variable abilities

[FikriMilano]

IMPORTANT: Where possible all PRs must be linked to a Github issue

Fixes #3111

Engineer Checklist

• I have written Unit tests for any new feature(s) and edge cases for bug fixes
• I have added any strings visible on UI components to the strings.xml file
• I have updated the CHANGELOG.md file for any notable changes to the codebase
• I have run ./gradlew spotlessApply and ./gradlew spotlessCheck to check my code follows the project's style guide
• I have built and run the FHIRCore app to verify my change fixes the issue and/or does not break the app
• I have checked that this PR does NOT introduce breaking changes that require an update to Content and/or

Code Reviewer Checklist

• I have verified Unit tests have been written for any new feature(s) and edge cases
• I have verified any strings visible on UI components are in the strings.xml file
• I have verifed the CHANGELOG.md file has any notable changes to the codebase
• I have verified the solution has been implemented in a configurable and generic way for reuseable components
• I have built and run the FHIRCore app to verify the change fixes the issue and/or does not break the app

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 29.6%. Comparing base (ac82739) to head (0eb833b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##              main   #3112   +/-   ##
=======================================
  Coverage     29.6%   29.6%           
  Complexity     658     658           
=======================================
  Files          239     239           
  Lines        11204   11204           
  Branches      1948    1948           
=======================================
  Hits          3323    3323           
  Misses        7447    7447           
  Partials       434     434           

Flag	Coverage Δ
engine	66.2% <ø> (ø)
geowidget	47.2% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

[ellykits]

@f-odhiambo Can you comment on this addition?

[f-odhiambo]

LGTM