To fix the func_wall, set its 'Minimum light level' (_minlight) to 0. It was set to 1, making it fullbright.
Indeed it was. But my problem is that the borders of the light are fullbright too. According to this tutorial (https://twhl.info/wiki/page/Tutorial%3A_In_the_Beginning_Part_4), if I set the brush as a func_wall, and give non-light textures to the borders, the borders shouldn't light up.
The func_water is trickier: apparently water textures (textures whose name starts with an exclamation mark) are always rendered fullbright by the engine. If you don't need the water surface to be wavy then you can use a non-water texture instead. Otherwise, you can set the func_water's 'Render Mode' to 'Texture', with a fairly low 'FX Amount'. That'll make the water more translucent, reducing its brightness against a dark background.
I see, it makes sense. I started to think this was the case when I remembered finding it strange that water is somewhat lit up in dark tunnels in Thief: The Dark Project. It came out the same year as Half-Life, so I thought it could be some sort of technical limitation of the time.
As for func_detail, it's a brush entity, so it won't show up in the list of point entities. But if you're turning a brush into an entity and still don't see func_detail in that list, then perhaps you've got an older zhlt.fgd file?
Ah, I was using the halflife.fgd file that came with J.A.C.K. Now I use the zhlt.fgd file and I now have func_detail in the editor. Thanks!