added a note in the docs about the fact that installing the C ext is optional
Update slice.rst
minor #1422 Update slice.rst (1emming)

This PR was merged into the 1.16-dev branch.

Discussion
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Update slice.rst

Please state that Twig `slice` is  multi-byte safe since it uses `mb_substr` :)

Commits
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87165e7 Update slice.rst