document and improve developer scripts

- markdown docs were intentionally held back from prior PR to avoid
  merge conflict
- add -help usage notes to all scripts
- add sanity check against user running develop_brew_cask inside
  /usr/local/Library/Taps/phinze-cask/developer/bin and better explain that
- note everywhere that "brew update" is not safe when in "development mode"
Merge pull request #2472 from rolandwalker/devel_scripts_followup

document and improve developer scripts