Auto merge of #6493 - agrim123:agr-update-bundle-update-docs, r=colby-swandale
Auto merge of #6493 - agrim123:agr-update-bundle-update-docs, r=colby-swandale

Add documentation for --all flag of bundle update

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

`bundle update` throws an error if run without any flags. `--all` flag was introduced to update all gems instead of no flags. This was not mentioned in docs.

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