Document known issues with HDR 2D alpha blending in Environment and post-processing

[Calinou]

• See 2D-HDR may cause issues with texture alpha rendering godot#80868
  and Enabling HDR 2D changes editor colors godot#80912.

[skyace65]

I know we generally don't like documenting bugs but I'm fine with it in this case, especially since it links to the issue so we'll know when it's been fixed. I'd like to get at least one other approval before a merge though.

[tetrapod00]

Soft approval. It's better than the status quo, and broadly I think it would be okay to link to known issues more than we currently do. We definitely should not close the linked issue.

In the documentation, IMO calling something a "known issue" implies that it is a bug and we would like to fix it. But the current discussion in godotengine/godot#80868 from maintainers indicates that this is a known limitation that should be documented, not a bug to be fixed. The issue is labeled discussion and documentation. The correct way to resolve that would be to rewrite some of the comments in that issue into a version that fits the manual. Instead, this PR links to the issue, essentially using godotengine/godot#80868 (comment) (and other comments) as documentation, rather than rewriting it into the manual format. Again, okay with me as better than status quo, but not ideal.

I'd also be okay with not merging, because this:

• links to an explanation rather than rewriting it into manual format, which is unusual.
• the linked issue is not a bug, so is unlikely to be resolved soon, and therefore isn't a great fit for a "known issue" link from the docs.

[skyace65]

After thinking this over some more I agree with tetrapod, though I'm giving this a hard no as is now. It should be explained as a limitation, and we shouldn't be linking to an engine issue, all info should be in the docs.