Add alternative considered and clarify motivation (#1639)
Add alternative considered and clarify motivation (#1639)

* Add alternative considered and clarify motivation

* Update 0354-regex-literals.md

* Update 0354-regex-literals.md

* Update proposals/0354-regex-literals.md

* Update regex literal proposal

Update link to syntax proposal, and clarify
terminology around "multi-line" and "extended".

Co-authored-by: Hamish Knight <c9643d17a553ab14be7750073a6378d6bfdb50de@mediocremail.com>
apple/swift-evolution,apple/swift-evolution