Document various QtConsole embedding approaches.

Also added loading of default config to the in-process embedding example.
Correct the explanations for embedding.
Revert config-loading change in embedding example.
Clarify embedding in IPython; mention 4th method.
Improve description of Qt console embedding options
Merge pull request #4939 from takluyver/embed-qtconsole-docs

Embed qtconsole docs (continued)