Clarify documentation workflows for assemblies with UseCompilerGeneratedDocXmlFile

[Copilot]

Adds clarification from dotnet/dotnet-api-docs#12000 (comment) about the two documentation workflows based on the UseCompilerGeneratedDocXmlFile property.

Key points:

• Assemblies with <UseCompilerGeneratedDocXmlFile>false</UseCompilerGeneratedDocXmlFile> (older assemblies, System.Private.CoreLib): dotnet-api-docs is source of truth; triple-slash comments sync once for new APIs only
• Assemblies without this property (newer assemblies): source comments are source of truth; synced continuously to dotnet-api-docs

Changes:

• adding-api-guidelines.md: Added "Documentation Workflow" subsection explaining both workflows with cross-reference to project-guidelines.md
• project-guidelines.md: Enhanced "XML Documentation Files" section with workflow implications and cross-reference to adding-api-guidelines.md

Both documents now consistently explain when and where to update documentation based on the assembly's configuration.

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Original prompt

Please create PR that adds addtional information from dotnet/dotnet-api-docs#12000 (comment) to adding-api-guidelines.md doc in this repo.

Also, reconsile it with the relevant section in project-guidelines.md

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[jkotas]

@ericstj Could you please review this - is this accurate description of the current process?

[Copilot]

Pull Request Overview

This PR updates the documentation workflow guidelines to provide clearer guidance on how API documentation is handled for different types of libraries. The key change is differentiating between libraries that use compiler-generated documentation (newer libraries) and those that rely on manually curated documentation in the dotnet-api-docs repository (older libraries).

Key changes:

• Restructured the documentation workflow section to explain two distinct paths based on the UseCompilerGeneratedDocXmlFile property
• Added explicit guidance on when triple-slash comments sync to dotnet-api-docs and when they serve as source of truth
• Updated cross-references between project guidelines and API guidelines documents

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
docs/coding-guidelines/project-guidelines.md	Replaced explanation of UseCompilerGeneratedDocXmlFile=false with a reference to the expanded documentation workflow section
docs/coding-guidelines/adding-api-guidelines.md	Expanded documentation section to distinguish between two workflows: compiler-generated docs (default) and manually curated docs, with clear guidance for each

[noahfalk]

Looks great 👍

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[stephentoub]

Looks right to me, thanks

[stephentoub]

/ba-g markdown only

[jkotas]

@ericstj I have tried to incorporate your suggestions. PTLA

[jkotas]

/ba-g doc only change