[docs] Emphasize that LibraryEvolution.rst is still in draft form.

Max pointed out that GitHub's ReST rendering doesn't treat ".. warning::"
specially.

Author: jrose-apple

docs/LibraryEvolution.rst

@@ -42,7 +42,7 @@ We also intend to provide tools to detect inadvertent changes in interfaces.
 
 .. contents:: :local:
 
-.. warning:: This document is still in draft stages. Large additions and
+.. warning:: **This document is still in draft stages.** Large additions and
   restructuring are still planned, including:
 
   * A summary for each declaration kind what changes are binary-compatible.