API docs: Remove note

That note doesn't belong here.
API docs: Add missing instructions on how to enable the API
Requirements: Provide a maximum acceptable release for Tastypie

Newer Tastypie releases do not support Django 1.4.x anymore.
Docs: Move 2.5.1 changelog to the corresponding release notes
Docs: Move 2.5.0 changelog to the corresponding release notes
Docs: Remove 2.1.6 from changelog

All data is already present in the release notes
Docs: Small cleanups

Partial cherry-pick for c4b627d8.
Remove stray os.close(fd)

A fallout from 2bc49e00d7485dabb5659c8104d5224ec1cad451.