Update include_relative tests

This verifies you can’t go “up” in the include
Add some more docs on `include_relative`
Set tag name as ivar

This lets us use just one syntax_example method for both classes.
Fixing "vertycal" typo

s/vertycal/vertical

Unless, like, "vertycal" was on purpose or something.
Merge pull request #2889 from pdehaan/patch-1
Update history to reflect merge of #2889 [ci skip]
Merge pull request #2884 from gjtorikian/improve-docs-for-include-relative
Update history to reflect merge of #2884 [ci skip]
Point Liquid links to Liquid’s Github wiki
Fixes #2883