contrib/hashicorp/vault: add godoc links

[dolmen]

What does this PR do?

Improve Go doc of package contrib/hashicorp/vault by using godoc links.

Motivation

Improved readability of the documentation thanks to cross links.

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• New code is free of linting errors. You can check this by running golangci-lint run locally.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

[rarguelloF]

Hi @dolmen,

Thanks for the PR, we really appreciate your contribution!

Currently, this PR cannot be merged because of the new repository requirement (Commits need to be signed). So if possible, could you:

• If you’re comfortable doing so, feel free to GPG-sign the commits yourself to comply with the repo’s commit policy.
• Otherwise, please enable the “Allow edits from maintainers” option on this PR on your fork so we can sign the commits on your behalf.

Thanks!