Update comparison with `sed`

[lambda]

Mention the -E flag for sed, but with some caveats, to make the comparison more fair. Split out the delimiter quoting from the slash quoting examples, and pick an example that's more favorable for sd. Add a simple example just comparing the noisier syntax of s///g vs two arguments. Add an example of the danger of sed -i in cross-platform usage.

[chmln]

Thanks!