[Sass] Document old-style properties.

doc-src/INDENTED_SYNTAX.md

@@ -70,6 +70,21 @@ and indentation instead of braces.
 However, there are some cases where there are differences or subtleties,
 which are detailed below.
 
+## Property Synax
+
+The indented syntax supports two ways of declaring CSS properties.
+The first is just like CSS, except without the semicolon.
+The second, however, places the colon *before* the property name.
+For example:
+
+    #main
+      :color blue
+      :font-size 0.3em
+
+By default, both ways may be used.
+However, the {file:SASS_REFERENCE.md#property_syntax-option `:property_syntax` option}
+may be used to specify that only one property syntax is allowed.
+
 ### Multiline Selectors
 
 Normally in the indented syntax, a single selector must take up a single line.