bootstrap readme: fix, improve, update

…kingjubilee

Rollup of 15 pull requests

Successful merges:

 - rust-lang#116791 (Allow codegen backends to opt-out of parallel codegen)
 - rust-lang#116793 (Allow targets to override default codegen backend)
 - rust-lang#117458 (LLVM Bitcode Linker: A self contained linker for nvptx and other targets)
 - rust-lang#119385 (Fix type resolution of associated const equality bounds (take 2))
 - rust-lang#121438 (std support for wasm32 panic=unwind)
 - rust-lang#121893 (Add tests (and a bit of cleanup) for interior mut handling in promotion and const-checking)
 - rust-lang#122080 (Clarity improvements to `DropTree`)
 - rust-lang#122152 (Improve diagnostics for parenthesized type arguments)
 - rust-lang#122166 (Remove the unused `field_remapping` field from `TypeLowering`)
 - rust-lang#122249 (interpret: do not call machine read hooks during validation)
 - rust-lang#122299 (Store backtrace for `must_produce_diag`)
 - rust-lang#122318 (Revision-related tweaks for next-solver tests)
 - rust-lang#122320 (Use ptradd for vtable indexing)
 - rust-lang#122328 (unix_sigpipe: Replace `inherit` with `sig_dfl` in syntax tests)
 - rust-lang#122330 (bootstrap readme: fix, improve, update)

r? `@ghost`
`@rustbot` modify labels: rollup

Rollup merge of rust-lang#122330 - tshepang:patch-1, r=clubby789

bootstrap readme: fix, improve, update