chore: escape brackets in documentation

[verpeteren]

Cargo doc was raising a warning like this:

warning: unresolved link to `i`
    --> src/lib.rs:1053:50
     |
1053 |     /// Note, SSE2 and SSE4 will store when mask[i] is nonzero, where AVX2
     |                                                  ^ no item named `i` in scope
     |
     = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`
     = note: `#[warn(rustdoc::broken_intra_doc_links)]` on by default