Improve wording of index-of and last-index-of doc strings

Feel free to decline this if it is too trivial. I was reviewing doc strings for new functions in Clojure 1.8.0, and those for clojure.string/index-of and clojure.string/last-index-of seem like they could be worded in a way that is much less subject to confusion.

Current doc string for clojure.string/index-of:

Issue: the lack of punctuation in the phrase "optionally searching forward from from-index or nil if not found" makes it appear that the "or" connects the first and last part of that phrase.

Suggested clarification:

Screened by: Alex Miller