Adding a section about the slice filter

This code example shows how to loop over a subset of values in order to implement one common use-case of "break" in PHP. I decided to use the "full" notation of slice rather than the sugared version (that example would just be "for user in users[:10]")
Adding link to slice filter per Stof's suggestion
merged branch beryllium/patch-1 (PR #1032)

This PR was merged into the master branch.

Discussion
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Adding a section about the slice filter to doc/tags/for.rst

This code example shows how to loop over a subset of values in order to implement one common use-case of "break" in PHP. I decided to use the "full" notation of slice rather than the sugared version (that example would just be "for user in users[:10]")

Commits
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202641a Adding link to slice filter per Stof's suggestion
8ebbaaa Adding a section about the slice filter
tweaked previous merge