Make The Compiling for Web documentation more obvious

I updated the documentation with workaround if it's not compiling from my experience in Bug #11197, and thanks to the help of @eska014.

development/compiling/compiling_for_web.rst

@@ -10,21 +10,31 @@ Requirements
 
 To compile export templates for the Web, the following is required:
 
--  `Emscripten SDK <http://emscripten.org/>`__ (Install in a path without
-   spaces, i.e. not on "Program Files")
--  `Python 2.7+ <https://www.python.org/>`__ (3.0 is
-   untested as of now)
+-  `Emscripten <http://emscripten.org/>`__, plus `LLVM <https://llvm.org/>`__ and `Binaryen <https://github.com/WebAssembly/binaryen>`__ if you want to compile for *Webassembly* (Godot 3). The easiest way to install them if they are not up to date in your Linux package manager is to use the `Emscripten SDK <http://emscripten.org/>`__ (Install in a path without spaces, i.e. not on "Program Files")
+-  `Python 2.7+ <https://www.python.org/>`__ (3.0 is untested as of now)
 -  `SCons <http://www.scons.org>`__ build system
 
 Building export templates
 -------------------------
 
 Start a terminal and set the environment variable ``EMSCRIPTEN_ROOT`` to the
-installation directory of Emscripten::
+installation directory of Emscripten:
+
+-  If you installed Emscripten via the Emscripten SDK, you will declare the variable with a path to your downloaded folder:
 
     export EMSCRIPTEN_ROOT=~/emsdk/emscripten/master
+    
+-  If you installed Emscripten via your package manager, you can know the path with the ``em-config`` command
+
+    em-config EMSCRIPTEN_ROOT
+
+   So you will declare the variable as for example:
 
-If you are on Windows, start a regular prompt or the Emscripten Command Prompt.
+    export EMSCRIPTEN_ROOT=/usr/lib/emscripten
+    
+The Emscripten variables are defined in the ``~/.emscripten`` file, so this is an alternative way to check. Erase this file if you want to reinstall Emscripten with a fresh new method.
+
+If you are on Windows and used Emscripten SDK, start a regular prompt or the Emscripten Command Prompt.
 Do **not** use the Developer Command Prompt nor any of the ones that come with
 Visual Studio. You can set the environment variable in the system settings or
 in the prompt itself::
@@ -43,6 +53,8 @@ the resulting file will be placed in the ``bin`` subdirectory. Its name is
 ``godot.javascript.opt.zip`` for release or ``godot.javascript.opt.debug.zip``
 for debug.
 
+If it's not, check if you have properly installed everything, and if EMSCRIPTEN_ROOT path is correctly defined, including in the ``~/.emscripten`` file.
+
 Finally, rename the zip archive to ``javascript_release.zip`` for the
 release template::
 
@@ -96,6 +108,8 @@ These commands will build WebAssembly export templates in either release or
 debug mode. The generated files' names contain ``.webassembly`` as an
 additional file suffix before the extension.
 
+If it's not compiling, check if you have properly installed everything, and if ``LLVM_ROOT`` and ``BINARYEN_ROOT`` variables are defined in the ``~/.emscripten`` file. You can define the ``BINARYEN_ROOT`` path manually in this file if you already installed it and want to avoid Scons to compile it from ports during template compilation.
+
 Finally, the WebAssembly templates are renamed to ``webassembly_release.zip``
 and ``webassembly_debug.zip``::