Improve command line help docs to have more information about permission flags

[ry]

When I run deno run --help I get a list of all the flags, but the docs are lack luster. There's an opportunity to provide examples and explanation under each of these. While the docs should be terse, it doesn't need to be limited to 5 words.

      --allow-read[=<allow-read>...]
          Allow file system read access

      --allow-write[=<allow-write>...]
          Allow file system write access

      --allow-net[=<allow-net>...]
          Allow network access

      --allow-env[=<allow-env>...]
          Allow environment access

      --allow-sys[=<allow-sys>...]
          Allow access to system info

      --allow-run[=<allow-run>...]
          Allow running subprocesses

      --allow-ffi[=<allow-ffi>...]
          Allow loading dynamic libraries

      --allow-hrtime
          Allow high resolution time measurement

[kwhinnery]

Hey there! Would love to take a crack at fixing this. Before I go too far though, I created a small draft PR with a proposed format for the changes. If that general approach looks good, I'm happy to continue updating the rest of the arguments for deno run (after which it might make sense to look at more arguments across the board).

Thanks!

[kwhinnery]

PR for this is ready for review! https://github.com/denoland/deno/pull/18757