[vk, renderdoc] fixed zero-sized streambuffer bug on old heap-absent GPUs (#4052)

This fix a bug in which eden crashes when renderdoc is attached to vulkan.
that kept me away from renderdoc for around a year now.

the bug:

in video_core\renderer_vulkan\vk_staging_buffer_pool.cpp
in GetStreamBufferSize(...)
if device.HasDebuggingToolAttached()
but heap is empty/unavailable <-- Case in my old nvidia kepler gpu

the original method returns size 0, right?
the change honors same original behavior, while covers my case properly, returning MAX_STREAM_BUFFER_SIZE.

addl some log tip and some minimal doc. fully safe. let it rip.

Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/4052
Reviewed-by: MaranBr <maranbr@eden-emu.dev>
Reviewed-by: Lizzie <lizzie@eden-emu.dev>

docs/README.md

@@ -12,6 +12,7 @@ This contains documentation created by developers. This contains build instructi
 - **[Development Guidelines](./Development.md)**
 - **[Dependencies](./Deps.md)**
 - **[Debug Guidelines](./Debug.md)**
+- **[RenderDoc usage](./RenderDoc.md)**
 - **[CPM - CMake Package Manager](./CPMUtil)**
 - **[Platform-Specific Caveats](./Caveats.md)**
 - **[The NVIDIA SM86 (Maxwell) GPU](./NvidiaGpu.md)**