Add examples for edge cases of str.split/str.splitn

In particular, show examples for splitting the empty string and using `splitn`
with a count of 0.

Fix #14222.
auto merge of #14335 : tbu-/rust/pr_doc_strsplit, r=pnkfelix

In particular, show examples for splitting the empty string and using `splitn`
with a count of 0.

Fix #14222.