Rollup merge of #80323 - camelid:codegen-base-docs, r=nagisa

Rollup merge of #80323 - camelid:codegen-base-docs, r=nagisa

Update and improve `rustc_codegen_{llvm,ssa}` docs

Fixes #75342.

These docs were very out of date and misleading. They even said that
they codegen'd the *AST*!

For some reason, the `rustc_codegen_ssa::base` docs were exactly
identical to the `rustc_codegen_llvm::base` docs. They didn't really
make sense, because they had LLVM-specific information even though
`rustc_codegen_ssa` is supposed to be somewhat generic. So I removed
them as they were misleading.

r? ``@pnkfelix`` maybe?