[Documentation] Add missing XML documentation for BlendState

[AristurtleDev]

Description

This PR adds the missing XML documentation for the Blendstate class.

Reference

Feature Request: Resolve Missing XML For Public Type Warnings

Note

This file has inconsistent line endings, which leads to issues when trying to update and keep the diff as focused as possible.

I manually went through to ensure no auto formatting or line ending normalization occurred to keep the diff small.

[AristurtleDev]

Switching to draft PR so reviews and feedback can be made, but waiting for guidelines to be updated for documentation before checking for merge

[AristurtleDev]

Ready for review now that contributing guidelines updated.