Fix link to AVX2 docs

[Pratyush]

Currently the library doesn't compile on nightly when the avx2_backend is enabled; this PR fixes it by fixing the links to the relevant docs.

[hdevalence]

Hi -- what nightly are you using? Recent nightlies changed the path root for doc(include) to refer to the crate root rather than the current file (cf. #269, rust-lang/rust#60938), and we updated the crate to match the current nightly behaviour. Is it still broken?

[Pratyush]

Hmm I am using the latest nightly. I tried changing the path to be "docs/avx2-notes.md" and "docs/parallel-formulas.md" and that didn't fix it:

➜ cargo +nightly --version
cargo 1.38.0-nightly (26092da33 2019-07-31)

➜ rustc +nightly --version
rustc 1.38.0-nightly (c4715198b 2019-08-05)

➜ cargo +nightly check --features avx2_backend
    Checking curve25519-dalek v1.2.2 (/tmp/curve25519-dalek)
error: couldn't read src/backend/vector/docs/parallel-formulas.md: No such file or directory (os error 2)
  --> src/backend/vector/mod.rs:22:19
   |
22 |     doc(include = "docs/parallel-formulas.md")
   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^ couldn't read file

error: couldn't read src/backend/vector/avx2/docs/avx2-notes.md: No such file or directory (os error 2)
  --> src/backend/vector/avx2/mod.rs:22:19
   |
22 |     doc(include = "docs/avx2-notes.md")
   |                   ^^^^^^^^^^^^^^^^^^^^ couldn't read file

error: aborting due to 2 previous errors

error: Could not compile `curve25519-dalek`.

[Pratyush]

Also, your links indicate that the new root is relative to the current file, not the crate root, so I think my fix makes sense by making the paths have the additional ../?

[hdevalence]

Uh oh, that's bad news, I thought it was already fixed.... let me dig into this

[hdevalence]

Yep, you're absolutely right, sorry -- I think that 5c18bfb didn't fix it, because I just added one .., but I'm confused why the CI didn't pick it up.

[hdevalence]

Hmm, the CI job did print an error, but it was reported as passing: https://travis-ci.org/dalek-cryptography/curve25519-dalek/jobs/566199718

[hdevalence]

@Pratyush thanks for spotting this -- could you amend the commit to also update to doc(include = "../../../../docs/ifma-notes.md") in src/backend/vector/ifma/mod.rs, or alternately give me access to push that commit to your branch (the "allow edits from maintainers" option), whichever is easier?

[Pratyush]

Force-pushed; should be fixed now, I think?

[hdevalence]

Great, I confirmed that running make doc-internal using cargo 1.38.0-nightly (26092da33 2019-07-31) / rustc 1.38.0-nightly (c4715198b 2019-08-05) produces the correct output.

[Pratyush]

Awesome =)