added missing backticks for legibility
named outlets, and rewritten warning message stuff

named outlets are referred to in the original, but not explained, so I
had to go on Stack Overflow to find out what they were. this version of
the doc gives a brief overview of named outlets. (in an ideal world I'd
also have an update for the handlebars template section. maybe one day.)

I rewrote the warning message stuff because I found it difficult to
understand. it also kind of just appeared out of nowhere in the middle
of a structure which leads you through the basics of template rendering,
so I moved it down to its own section.
Merge pull request #440 from gilesbowkett/less_confusing_explanation_of_rendering_warning

Described named outlets, and rewrote explanation of rendering warning