doc: add a note about the empty regex
doc: add a note about the empty regex

This improvement is a consequence of the Q&A discussion from
https://github.com/rust-lang/regex/discussions/896

PR #897 
rust-lang/regex,rust-lang/regex,rust-lang-nursery/regex,rust-lang-nursery/regex,rust-lang/regex,rust-lang/regex,rust-lang-nursery/regex,rust-lang/regex,rust-lang-nursery/regex,rust-lang-nursery/regex