[clang] [llvm] [clang] Add/enhance documentation for some important classes. (PR #109795)

[Sam McCall via cfe-commits]

sam-mccall wrote:

There are some classes here where I think the concepts are tricky and under-explained (SourceManager, TypeLoc) or just important enough that it's worth spending a few more words in general.
(I'm sure there are a bunch more, but I hit my timebox)

If you think anything is too wordy/confusing or I'm removing anything important, please do say so!

Let me know if it's better to split these into smaller patches.

https://github.com/llvm/llvm-project/pull/109795