[Docs] Explain display modes and internal resolution limitations

docs/rendering-resolution.md

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+<!--
+Copyright (C) 2026 SharpEmu Emulator Project
+SPDX-License-Identifier: GPL-2.0-or-later
+-->
+
+# Rendering resolution
+
+Options → Rendering exposes two independent sizes, globally and per game:
+
+- **Resolution** sets the host window size or exclusive fullscreen mode.
+- **Game display mode** reports a 1080p display to the game by default. Select
+  3840×2160 to request a 4K display. A registered presentation buffer cannot
+  change this setting.
+
+The CLI equivalents are `--resolution=1280x720` and
+`--guest-resolution=1920x1080` (or `3840x2160`). For example, a 1080p game display
+can be presented in a 720p window; changing the window alone does not reduce the
+work the game renders.
+
+The game chooses its internal render targets. Some games adapt those targets to
+the reported display mode; others render at a fixed resolution and may still
+register 4K buffers with a 1080p display. Silent Hill: The Short Message does so
+in current testing. The size of that presentation buffer alone also does not
+establish the sizes of its intermediate scene targets.
+
+Arbitrary internal resolution scaling is not implemented. The previous
+`RenderResolutionScale` setting and its `SHARPEMU_RENDER_SCALE` export had no
+renderer consumer; they have been removed. They did not change rendering work.
+A correct implementation must preserve guest memory layouts while scaling host
+attachments, viewports, scissors, image fetches, compute accesses, aliases and
+readbacks consistently. Reducing only the output image or window is not an
+internal-resolution implementation.