Fixed functions doc

You wouldn't want your user to use Twig's tag syntax (`{% %}`) for functions (`{{ }}`), as they would have some mysterious errors (`[LogicException] Could not process "xxx": "Unknown tag name "yyy" in "__string_template__zzz" at line aaa"`, `[Twig_Error_Syntax] Unknown tag name "yyy" in "__string_template__aaa" at line 3`)
bug #24 Fixed functions doc (gnugat)

This PR was merged into the 2.0.x-dev branch.

Discussion
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Fixed functions doc

You wouldn't want your user to use Twig's tag syntax (`{% %}`) for functions (`{{ }}`), as they would have some mysterious errors (`[LogicException] Could not process "xxx": "Unknown tag name "yyy" in "__string_template__zzz" at line aaa"`, `[Twig_Error_Syntax] Unknown tag name "yyy" in "__string_template__aaa" at line 3`)

Commits
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bdee708 Fixed functions doc