Clarify and expand on GET /_session response

[flimzy]

Overview

Grammar clarifications, and expanding of response details for GET /_session documentation.

Checklist

• Documentation is written and is accurate;
• make check passes with no errors
• Update rebar.config.script with the commit hash once this PR is rebased and merged

[nickva]

+1

[flimzy]

I thought I had access to merge my own PRs now, but apparently I don't yet (any longer?) so I guess I'll need someone else to merge this for me :)

[wohali]

@flimzy you need to visit https://gitbox.apache.org/ to link your accounts, and wait about an hour. Give it a shot.

[nickva]

@flimzy I won't merge it yet, let's use this as a test to see if you can merge after the gitbox setup.

[flimzy]

I've got the button now. Thanks for the patience, @wohali & @nickva