Document and test the ${{{}}:...} and ${...:{{}}} edge cases.

Test the latter, and document how to work around the former.

slapos/recipe/template/README.jinja2.txt

@@ -40,6 +40,7 @@ And according Jinja2 template (kept simple, control structures are possible)::
     ...     'Knights who say "{{knight}}" also protect {{ words | join(", ") }}.\n'
     ...     'They later say {{later}}\n'
     ...     '${this:is_literal}\n'
+    ...     '${foo:{{bar}}}\n'
     ...     'swallow: {{ json_module.dumps(("african", "european")) }}\n'
     ...     'parameters from section: {{ param_dict | dictsort }}\n'
     ...     'Rendered with {{recipe}}'
@@ -57,6 +58,7 @@ And the template has been rendered::
     Knights who say "Ni !" also protect Peng, Neee-wom.
     They later say Ekke Ekke Ekke Ekke Ptangya Ziiinnggggggg Ni!
     ${this:is_literal}
+    ${foo:value}
     swallow: ["african", "european"]
     parameters from section: [('bar', 'bar'), ('foo', '1')]
     Rendered with slapos.recipe.template:jinja2
@@ -118,6 +120,16 @@ Optional:
   Jinja2 extensions to enable when rendering the template,
   whitespace-separated. By default, none is loaded.
 
+FAQ
+---
+
+Q: How do I generate ${foo:bar} where foo comes from a variable ?
+
+A: ``{{ '${' ~ foo_var ~ ':bar}' }}``
+   This is required as jinja2 fails parsing "${{{ foo_var }}:bar}". Though,
+   jinja2 succeeds at parsing "${foo:{{ bar_var }}}" so this trick isn't
+   needed for that case.
+
 Use jinja2 extensions
 ~~~~~~~~~~~~~~~~~~~~~