doc: documentations for `getglobal`/`setglobal!`

[aviatesk]

Those new builtins are added in #44231 and will be exported at v1.9,
but currently it seems like we don't have documentations for them.

/cc @simeonschaub

[wolthom]

@aviatesk Is this issue still relevant? If so, are there tests I should refer to regarding the functionality / edge cases?