Make MB aware of iOS bot configs and get iOS working.

tools/mb/docs/user_guide.md

 # The MB (Meta-Build wrapper) user guide
 
 [TOC]
 
 ## Introduction
 
 `mb` is a simple python wrapper around the GYP and GN meta-build tools to
 be used as part of the GYP->GN migration.
 
 It is intended to be used by bots to make it easier to manage the configuration
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 a directory, then runs GYP or GN as appropriate:
 
 ```
 % mb gen -m tryserver.chromium.linux -b linux_rel //out/Release
 % mb gen -c linux_rel_trybot //out/Release
 ```
 
 Either the `-c/--config` flag or the `-m/--master` and `-b/--builder` flags
 must be specified so that `mb` can figure out which config to use.
 
-By default, MB will look in `//tools/mb/mb_config.pyl` to look up the config
-information, but you can specify a custom config file using the
-`-f/--config-file` flag.
+By default, MB will look for a bot config file under `//ios/build/bots` (see
+[design_spec.md](the design spec) for details of how the bot config files
+work). If no matching one is found, will then look in
+`//tools/mb/mb_config.pyl` to look up the config information, but you can
+specify a custom config file using the `-f/--config-file` flag.
 
 The path must be a GN-style "source-absolute" path (as above).
 
 You can pass the `-n/--dryrun` flag to mb gen to see what will happen without
 actually writing anything.
 
 You can pass the `-q/--quiet` flag to get mb to be silent unless there is an
 error, and pass the `-v/--verbose` flag to get mb to log all of the files
 that are read and written, and all the commands that are run.
 
@@ skipping 147 matching lines @@
 run than you expect; running `'mb -v'` will print what it's doing and
 run the commands; `'mb -n'` will print what it will do but *not* run
 the commands.
 
 If you hit weirder things than that, add some print statements to the
 python script, send a question to gn-dev@chromium.org, or
 [file a bug](https://crbug.com/new) with the label
 'mb' and cc: dpranke@chromium.org.