doc: add a subsection on skipping examples.

* doc/specl.md (Skipping Examples): New subsection demonstrating
how to skip examples programatically.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
doc: correct argument order for error matcher.

The order of error matcher arguments was reversed with Specl 5.
* doc/specl.md (Pending Examples): Fix error matcher argument
order to match contemporary usage.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>