docs: Clarify install instructions

`config-clang` is the default, and doesn't need to be run first.  Previous instructions were ambiguous about that point.
Add note on using a different `CXX`.

docs/source/getting_started/installation.rst

@@ -110,26 +110,33 @@ Installing all prerequisites for macOS 11 (with Homebrew):
 Running the build system
 ^^^^^^^^^^^^^^^^^^^^^^^^
 
-To configure the build system to use a specific compiler, use one of the
-following:
-
-.. code:: console
-
-   make config-clang
-   make config-gcc
-
-Then, simply run ``make`` in this directory.
+From the root `yosys` directory, call the following commands:
 
 .. code:: console
    
    make
    sudo make install
 
-Note that this also downloads, builds, and installs `ABC`_ (using
+This will build and then install Yosys, making it available on the command line
+as `yosys`.  Note that this also downloads, builds, and installs `ABC`_ (using
 :program:`yosys-abc` as the executable name).
 
 .. _ABC: https://github.com/berkeley-abc/abc
 
+The default compiler is ``clang``, to change between ``clang`` and ``gcc``, use
+one of the following:
+
+.. code:: console
+
+   make config-clang
+   make config-gcc
+
+To use a compiler different than the default, use:
+
+.. code:: console
+
+   make CXX="g++-11"
+
 .. seealso:: 
 
    Refer to :doc:`/test_suites` for details on testing Yosys once compiled.