docs: clean up environment variables formatting & grammar

[kofi-bhr]

I noticed a bunch of inconsistencies in the capitalization and some minor grammar issues while browsing through the environment variables docs. Nothing that would break anything, but it was bugging me enough to fix it on a Monday afternoon :)

The changes are pretty straightforward:

• Standardized capitalization for things like "host", "port", "username" - little details that make the docs look more professional
• Fixed a few grammar issues like "it only need to be set" → "it only needs to be set"
• Removed that duplicate "use" in "to use use a Docker secret"
• Made the formatting more consistent with things like "comma-separated" and "CPU"

These are the kinds of small polish items that individually don't matter much, but collectively make documentation feel more professional and easier to read. Just my small contribution to help out - figured it was a decent way to spend 15 minutes while waiting for my build to finish on another project.