Documentation update.

llvm-svn: 74907

llvm/tools/llvmc/doc/LLVMC-Reference.rst

@@ -347,6 +347,12 @@ separate option groups syntactically.
      3))``. Only list options can have this attribute; you can, however, use
      the ``one_or_more`` and ``zero_or_one`` properties.
 
+   - ``init`` - this option has a default value, either a string (if it is a
+     parameter), or a boolean (if it is a switch; boolean constants are called
+     ``true`` and ``false``). List options can't have this attribute. Usage
+     examples: ``(switch_option "foo", (init true))``; ``(prefix_option "bar",
+     (init "baz"))``.
+
    - ``extern`` - this option is defined in some other plugin, see below.
 
 External options