[llvm] [LangRef] Rework DIExpression docs (PR #153072)

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

> I squashed and rebased, fixing up the relevant bits, and then extended the `inline-code`-ification from those patches through the entire `SourceLevelDebugging.rst` document. I added @kazutakahirata and @tgymnich in case they wanted to review. If I don't hear from them I'll probably merge early next week.

Thank you for looping me in and rebasing your PR.  I don't have specific inputs, so don't let me block you.  Thanks again!

https://github.com/llvm/llvm-project/pull/153072