Auto merge of #6271 - nholden:bundle-doctor-man-page, r=colby-swandale

Add man page for `bundle doctor`

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

The `bundle doctor` command was missing a man page (see #6243).

### What was your diagnosis of the problem?

Running `bundle help doctor` from the command line showed [the description from `cli.rb`](https://github.com/bundler/bundler/blob/723608f45866cee0f1b315551a8dde6a99efebc6/lib/bundler/cli.rb#L611-L620), but it wasn't displayed in the same format as commands with man pages. Documentation for `bundle doctor` was also missing from http://bundler.io/docs.

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

I added a man page for `bundle doctor`.

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

I chose this fix because the ["Writing docs for man pages" documentation](https://github.com/bundler/bundler/blob/master/doc/documentation/WRITING.md#what-goes-in-man-pages) says...

> Our goal is to have a man page for every command.

(cherry picked from commit b26714b)

man/bundle-doctor.ronn

@@ -0,0 +1,33 @@
+bundle-doctor(1) -- Checks the bundle for common problems
+=========================================================
+
+## SYNOPSIS
+
+`bundle doctor` [--quiet]
+                [--gemfile=GEMFILE]
+
+## DESCRIPTION
+
+Checks your Gemfile and gem environment for common problems. If issues
+are detected, Bundler prints them and exits status 1. Otherwise,
+Bundler prints a success message and exits status 0.
+
+Examples of common problems caught by bundle-doctor include:
+
+* Invalid Bundler settings
+* Mismatched Ruby versions
+* Mismatched platforms
+* Uninstalled gems
+* Missing dependencies
+
+## OPTIONS
+
+* `--quiet`:
+  Only output warnings and errors.
+
+* `--gemfile=<gemfile>`:
+  The location of the Gemfile(5) which Bundler should use. This defaults
+  to a Gemfile(5) in the current working directory. In general, Bundler
+  will assume that the location of the Gemfile(5) is also the project's
+  root and will try to find `Gemfile.lock` and `vendor/cache` relative
+  to this location.