xWebConfigKeyValue: Updated schema.mof to include description for Ensure

[mcbobke]

Pull Request (PR) description

Updated schema.mof for xWebConfigKeyValue to include description for Ensure property.

This Pull Request (PR) fixes the following issues

• Fixes xWebConfigKeyValue: Missing description on property Ensure #455

Task list

• Added an entry under the Unreleased section of the change log in the CHANGELOG.md.
  Entry should say what was changed, and how that affects users (if applicable).
• Resource documentation added/updated in README.md.
• Resource parameter descriptions added/updated in README.md, schema.mof
  and comment-based help.
• Comment-based help added/updated.
• Localization strings added/updated in all localization files as appropriate.
• Examples appropriately added/updated.
• Unit tests added/updated. See DSC Resource Testing Guidelines.
• Integration tests added/updated (where possible). See DSC Resource Testing Guidelines.
• New/changed code adheres to DSC Resource Style Guidelines and Best Practices.

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This change is 

[codecov-io]

Codecov Report

Merging #505 into dev will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #505   +/-   ##
=======================================
  Coverage   91.39%   91.39%           
=======================================
  Files          17       17           
  Lines        2661     2661           
=======================================
  Hits         2432     2432           
  Misses        229      229

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[johlju]

Reviewed 1 of 2 files at r1, 1 of 1 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved