Make documentation pass with `strict = true` again.

[KristofferC]

Since #25706 the documentation no longer passes with strict = true.
This is because #25706 promoted some warnings:

 !! Invalid local link: unresolved path
    '../../devdocs/libgit2/#Base.LibGit2.GitRevWalker' in devdocs/libgit2.md
 !! Invalid local link: unresolved path
    '../../devdocs/libgit2/#Base.LibGit2.GitHash' in devdocs/libgit2.md
 ...

to errors

!! No doc found for reference '[`GitRevWalker`](@ref)'. [src/base/collections.md]
 !! No doc found for reference '[`GitHash`](@ref)'. [src/base/collections.md]
...

This seems to be due to referring to e.g. GitRevWalker in the LibGit2 extended method Base.map. I am not really sure why this happens though. One way to "solve" this would be to decouple Base.map and LibGit2.map similar to #24837.