Auto merge of #5417 - seban:document-mirror-fallback-timeout-option, r=segiddins
Auto merge of #5417 - seban:document-mirror-fallback-timeout-option, r=segiddins

Add documentation for mirror.fallback_timeout option.

As suggested in https://github.com/bundler/bundler/pull/5403#issuecomment-278384552 instead of changing parsing `mirror.fallback_timeout` I added documentation for it.
Pull request https://github.com/bundler/bundler/pull/5403 can be closed.

(cherry picked from commit f23034bf4c48a69fd2250cd7717eeedb4ac85216)

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