stringfilter documentation example does not work

[Antti Kaihola]

In ​stringfilter decorator documentation, the first code example given does not work because @register.filter is missing.

[Antti Kaihola]

Adds the missing @register.filter decorator to the @stringfilter example

[Gabriel Hurley]

Changing the docs as suggested won't work because we haven't been introduced to registering filters yet (and we'd have to add that line to all of the preceding examples). Instead, flipping the sections on "Registering custom filters" and "Template filters that expect strings" would make far more sense. That way the flow of information goes from writing filters to registering them to other useful information about them.

[hahasee]

I switched the sections and made the stringfilter example work. I wasn't sure if this was the place for a full example with all the includes, but wasn't sure what else to do.

[Chris Beaven]

In [16251]:

Fixes #15732 -- better introduction of template filter decorator concepts (first register, then stringfilter). Thanks for the patch, hahasee.

[Chris Beaven]

I removed @stringfilter from the register doc examples too, so we're introducing one concept at a time.