docs(util-fns): generalize getExampleName()

[chalin]

For some chapters the Jade source file name (e.g., toh-pt6.jade) does not match with the example directory name (toh-6). For such cases, we can now set _example to, say toh-6; getExampleName() will now use _example if set, otherwise determine the name from current as it did before.

[chalin]

@Foxandxss : ready for review and merge.
cc @kwalrath

[Foxandxss]

So with this, we can define a _example variable in the .jade so we can do better on the example paths through the jade?

[chalin]

Yes. E.g., toh-pt6.jade will soon have:

- var _example = 'toh-6';

at the top. Then the functions and mixins in the _utls-fns.jade files can uniformly call getExapleName() without concern of the means by which the name is obtained.

[chalin]

Subsumed by #1686 (since this was a necessary change for toh-6).