The GitLab::Commits::commit_discussions() introduced in c791a9f2 is
generalised in this commit to provide epic, commit, issue, merge request
and snippet discussions.

Array or hash parameters require '[]' suffix when passed to API. This
commit fixes problems when the suffix was not added for other than GET
requests (notably POST and PUT).

Perl POD documentation for these modules is virtually impossible to
maintain and is basically just copied from Web sources, which are
referenced in said Perl POD anyway.