Auto merge of #5934 - arbonap:pa-pristine-docs, r=colby-swandale
Auto merge of #5934 - arbonap:pa-pristine-docs, r=colby-swandale

Add more explanations to `bundle pristine`

Thanks so much for the contribution!
To make reviewing this PR a bit easier, please fill out answers to the following questions.

### What was the end-user problem that led to this PR?

The problem was...
- I didn't know in what situations it was practical to use `bundle pristine`.

### What was your diagnosis of the problem?

My diagnosis was...
- To write more docs for [bundler.io](bundler.io) and the man pages

### What is your fix for the problem, implemented in this PR?

My fix...
- To add more documentation regarding what practical situations a developer can use `bundle pristine`.
- Also, I added some explanation between the differences between `bundle pristine` and `gem pristine --all`
- Added a note that `pristine` doesn't work for local gems

### Why did you choose this fix out of the possible options?

I chose this fix because...
- Hopefully with a practical example of when to use `bundle pristine`, bundler's users can use this command more ✨

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