Improve docs for each compiler, closes #4099

Author: José Valim

lib/mix/lib/mix/tasks/compile.erlang.ex

@@ -41,6 +41,12 @@ defmodule Mix.Tasks.Compile.Erlang do
 
       For a list of the many more available options,
       see [`:compile.file/2`](http://www.erlang.org/doc/man/compile.html#file-2).
+
+  For example, to configure the `erlc_options` for your Erlang project you
+  may run:
+
+      erlc_options: [:debug_info, {:i, 'path/to/include'}]
+
   """
 
   @doc """

lib/mix/lib/mix/tasks/compile.ex

@@ -6,8 +6,7 @@ defmodule Mix.Tasks.Compile do
   @moduledoc """
   A meta task that compiles source files.
 
-  It simply runs the compilers registered in your project. At
-  the end of compilation it ensures load paths are set.
+  It simply runs the compilers registered in your project.
 
   ## Configuration
 
@@ -28,6 +27,18 @@ defmodule Mix.Tasks.Compile do
       this has undesirable side-effects (such as skipping some
       compiler checks) and should be avoided.
 
+  ## Compilers
+
+  To see documentation for each specific compiler, you must
+  invoke `help` directly for the compiler command:
+
+      mix help compile.elixir
+      mix help compile.erlang
+
+  You can get a list of all compilers by running:
+
+      mix compile --list
+
   ## Command line options
 
     * `--list`          - list all enabled compilers