Auto merge of #14281 - mbrubeck:mach-doc-fix, r=Manishearth

Run cargo doc in the ports/servo directory

Running it in components/servo isn't guaranteed to work because there's no Cargo.lock checked in there.  Followup to #14254.

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ports/servo/Cargo.toml

@@ -10,7 +10,6 @@ publish = false
 name = "servo"
 path = "main.rs"
 test = false
-doc = false
 bench = false
 
 [dev-dependencies]

python/servo/post_build_commands.py

@@ -217,7 +217,7 @@ class PostBuildCommands(CommandBase):
                         copy2(full_name, destination)
 
         return call(["cargo", "doc"] + params,
-                    env=self.build_env(), cwd=path.join('components', 'servo'))
+                    env=self.build_env(), cwd=self.servo_crate())
 
     @Command('browse-doc',
              description='Generate documentation and open it in a web browser',