hc-generate: Change default template to rust-proc and improve documentation

[timotree3]

PR summary

This PR

• Changes the default template used by hc generate to rust-proc (1e19808)
  • and in so doing, fixes holochain/rust-proc-zome-template#5
• Clarifies the documentation of the template argument used by hc generate (3d5864f)
• Improves the formatting of the documentation for hc generate (3bf5f92)
  • by moving some of it to the second line so that it only shows up in long help messages

testing/benchmarking notes

N/A

followups

N/A

changelog

• [x] if this is a code change that effects some consumer (e.g. zome developers) of holochain core, then it has been added to our between-release changelog with the format

- summary of change [PR#1234](https://github.com/holochain/holochain-rust/pull/1234)

documentation

• [x] this code has been documented according to our docs checklist

[zippy]

@freesig please give this a review to confirm it makes sense to switch the default based on current docs.