Clarify lintr hook documentation and fix argument names

[BjarkeHautop]

Arguments were listed with - instead of _, which is inconsistent with the actual hook implementation (see lintr.R).

This also clarifies what the --load_package argument does and when it should be used.

As an improvement, the default hook generation could include --load_package for {lintr} by default, and suggest that users call snippet_generate("additional-deps-lintr"), similar to how the roxygenize hook works.

For context, see the related issue: #647.

[BjarkeHautop]

snippet_generate("additional-deps-lintr") should probably also include suggested dependencies, so {lintr} works on readme, vignettes, and articles.

[lorenzwalthert]

Thanks.