various minor changes to the documentation (#50879)
A small contribution to an improved documentation. I've bundled a few
tiny, unrelated changes together instead of submitting many PRs. Let me
know if you prefer it differently or if you want any other change.
Some comments:
`llvmcall`: The file `test/llvmcall.jl` was mentioned. I've added a link
to GitHub. However, it works only for tagged version. Improvements
welcome.
`@sync`: I would like to add links to `Distributed.@spawnat` and
`Distributed.@distributed`, but I don't know how to do it from one
package (Base) to another. I once asked about it on
[Discourse](https://discourse.julialang.org/t/documentation-how-to-add-cross-references-to-functions-in-other-modules/88504),
but didn't get an answer.
`hasfastin`: now mentioned in the documentation
