This PR adds "Constant Special Item ID List (CSIDL)" terminology to the Environment.SpecialFolder enum documentation to improve discoverability when users search for CSIDL values.

Changes Made

• Updated the summary description from "enumerated constants" to "enumerated Constant Special Item ID List (CSIDL) values"
• This change is consistent with existing CSIDL references in the related Environment.xml file

Problem Addressed

The original documentation didn't include the widely-used "CSIDL" term, making it difficult for users to find this page when searching for:

• "what is CSIDL 36?"
• "CSIDL values"
• "Constant Special Item ID List"

Investigation Notes

Table Column Headers: The issue also mentioned changing the table column header from "Value" to "CSIDL Value". After thorough investigation, this is not possible through XML modifications because:

1. Column headers are auto-generated by the DocFX/ECMA2Yaml build system
2. No other enum files in the repository show custom column headers
3. Headers appear to be hardcoded in the documentation theme templates
4. The XML schema doesn't provide attributes for table presentation customization

The primary goal of improving discoverability is achieved through the summary change, which will make the page much more findable for CSIDL-related searches.

Fixes #11443.

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