Add cross-references from env var to CLI options documentation (for issue #171)

This pull request adds and cleans up documentation cross-references from the `env` variables doc to the docs for their respective command-line options, and resolves #171.

The pull request revives most of the changes of [a previously closed pull request][1]. As discussed there, these changes are now based on the version 1.5 branch, instead of master. (For what it's worth, I experimented with porting the changes to version 1.6 and master with `git cherry-pick` and it seemed to work cleanly, though today was the first time I've used `cherry-pick`, so I'm not willing to make any guarantees.)

Two intended formatting fixes were omitted from this request because they applied only to the 1.6 branch; I'll submit those fixes in a separate pull request. Finally, I wasn't entirely sure about where a changelog entry should go (branch or page order), so I didn't do that either, but I'm willing to accept direction on the matter.

Please let me know if there's anything else I can to do to get this right. Thanks!

[1]: https://github.com/fabric/fabric/pull/891