Improve path documentation.

REF: https://github.com/bundler/bundler/issues/3214
Add comment about bundler-features repo

Make it more obvious that feature discussions should go on the bundler-feature repo
Amend docs with block to discourage global sources for gems.

REF: https://github.com/bundler/bundler/issues/3214
Merge pull request #3264 from agenteo/3214-improve-path-documentation

Improve path documentation.
Merge pull request #3284 from GeekOnCoffee/patch-1

Add comment about bundler-features repo

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