improve split() docs
improve split() docs

Summary:
From reading the docs, I assumed that casting exceptions would be caught.  Let's clarify this.

Test Plan: readingiskey

Reviewed By: yfeldblum@fb.com

Subscribers: folly-diffs@, yfeldblum, chalfant

FB internal diff: D1960186

Signature: t1:1960186:1427920471:b7aa2af38a90a986418b98294fc0f69e678f0d1e

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