hadrian: improve user settings documentation

This patch adds minor improvements to hadrian user settings documentation:

• Add missing ghc.cpp.opts case
• Remove non-existent cxx case
• Clarify cc.c.opts also works for C++, while cc.deps.opts doesn't
• Add example of passing configure argument to autoconf packages

Fixes #24775 (closed). Also, the indentation level mismatch in the source text is intentional to fix rendering of nested bullet point list; if someone knows what's going on in markdown I'd be curious to know!