Convert pristine command desc to long_desc
Convert pristine command desc to long_desc

The default command description for `bundle pristine` was way too long
for simple description, and can trail off far beyond when called from
`bundle cli_help`.

That said, this is perfect info for a `long_desc`, so the current
content has been moved there, and the regular desc was a trimmed version
of that.

mvz/bundler,mvz/bundler,dilumnavanjana/bundler,dilumnavanjana/bundler,bundler/bundler,bundler/bundler