Expand folly::split documentation examples

Summary:
Documentation is likely a second (after code usage  in the codebase) if not the
first one source of usage examples. Let's expand it a little bit by adding more
usage examples:
* there is a difference between char (' ') and string (" ") delimiter
  performance wise.
* `folly::small_vector` and `folly::StringPiece` might be used to avoid heap
  allocation if number of split pieces are know in advance.

Reviewed By: IosifSpulber

Differential Revision: D50009704

fbshipit-source-id: 1b42102523b2a9a228e16ccb1115044463d9dcaa

folly/String.h

@@ -479,10 +479,13 @@ struct IsSimdSupportedDelim<char> : std::true_type {};
  * Examples:
  *
  *   std::vector<folly::StringPiece> v;
- *   folly::split(":", "asd:bsd", v);
+ *   folly::split(':', "asd:bsd", v);
+ *
+ *   folly::small_vector<folly::StringPiece, 3> v;
+ *   folly::split(':', "asd:bsd:csd", v)
  *
  *   std::set<StringPiece> s;
- *   folly::splitTo<StringPiece>(":", "asd:bsd:asd:csd",
+ *   folly::splitTo<StringPiece>("::", "asd::bsd::asd::csd",
  *    std::inserter(s, s.begin()));
  *
  * Split also takes a flag (ignoreEmpty) that indicates whether adjacent