Request: Document default regex flags

[AndydeCleyre]

The --help output mentions the regex flags available, but not which ones are enabled by default. It could use a bit like "By default, acts like sd -f ce"

[balupton]

ripgrep and sd both use the rust regex implementation, so ripgreps flags should be the same as those here:

https://docs.rs/regex/latest/regex/

you can also use this syntax to enable flags locally:

(?flags)       set flags within current group
(?flags:exp)   set flags for exp (non-capturing)