[Docs Update] Regex syntax/features undocumented

[dabrahams]

Description

There are many regular expression languages in common use, with different features and syntax details. I want to know which features I can use and how to use them.

Page(s)

https://docs.codecov.com/docs/path-regexp#regexp-patterns

Current State

[] Broken Link
[] Unclear Language
[] Inconsistent Formatting
[x] Missing Information
[] Missing Updates for New Features
[] Deprecated Features