steer away from using `@cask` instance variable in `caveats` interpolation

Several Casks were damaged in #6533 as I changed `caveats` to the simpler string form, but forgot the fine points of how interpolation works at evaluation time *vs* install time.  This PR addresses that by

* changing `#{@cask}` interpolations to `#{title}` in 16 damaged Casks
* removing needless references to `@cask` such as `#{@cask.version}` interpolation in 2 Casks (just `#{version}` will do)
* updating docs to
  * steer away from install-time method interpolations (this form was previously featured first)
  * suggest placing `caveats` last so that info is filled in at evaluation time
  * update list of available interpolations
  * incidental updates to caveats section
* moving `caveats` to last position in 13 Casks

In many cases where `caveats` was not already in the last position, `manual_installer` was used, reinforcing the recent decision that `manual_installer` is actually a pseudo-artifact which must be hoisted out of `caveats` entirely.

The docs are somewhat complicated by the fact that `@cask` is not a proper instance, but an instance variable which refers to a class.  We sometimes lie a little bit and pretend Casks are instances, which is also a medium-term implementation plan.