docs: clarify and cleanup the certificate format spec

Also, cleanup references to the legacy signed interest spec

Change-Id: Ib761399b6e44cefea6331e4e128bb2976c2fc11d
diff --git a/docs/INSTALL.rst b/docs/INSTALL.rst
index 4ad0fba..dda1924 100644
--- a/docs/INSTALL.rst
+++ b/docs/INSTALL.rst
@@ -57,9 +57,12 @@
     * Install either Xcode (from the App Store) or the Command Line Tools
       (with ``xcode-select --install``)
 
-    * If using Homebrew (recommended), enter the following in a terminal::
+    * If using Homebrew (recommended), enter the following in a terminal:
+
+      .. code-block:: sh
 
         brew install boost openssl pkg-config
+        brew install python  # only on macOS 10.14 and earlier
 
       .. warning::
 
@@ -96,7 +99,7 @@
 
   .. code-block:: sh
 
-    sudo dnf config-manager --enable PowerTools  # on CentOS only
+    sudo dnf config-manager --enable powertools  # on CentOS only
     sudo dnf install doxygen graphviz python3-pip
     pip3 install --user sphinx sphinxcontrib-doxylink