Demote error-demonstrating doctest block

This is so that make doctest will pass again if you are not using a user account called sabae on a Mac.
Better documentation for gcdx
stdlib/base docs: convert examples to doctests

The standard library documentation contains many examples which can be processed as doctests.

Also includes formatting fixes:
- correctly wrap code references in double backquote environments
- Better cross-referencing of functions

Additionally, include some content fixes:

- A better description of gcdx
- Fixed embarrassing errors in iseven and isodd documentation. Contrary to the previous version of the docs, claimed that 9 is not even and 10 is not odd.
Merge pull request #7973 from JuliaLang/cjh/docfix

Convert examples to doctests in stdlib docs