Add an example to clang attr doc

Author: mysterymath

clang/include/clang/Basic/AttrDocs.td

@@ -9622,10 +9622,18 @@ not understand a aspect, it must summarily report that the format string has
 that aspect.
 
 The compiler reports an aspect by issing a relocation for the symbol
-`<impl_name>_<aspect>``. This arranges for code and data needed to support the
+``<impl_name>_<aspect>``. This arranges for code and data needed to support the
 aspect of the implementation to be brought into the link to satisfy weak
 references in the modular implemenation function.
 
+For example, say ``printf`` is annotated with
+``modular_format(__modular_printf, __printf, float)``. Then, a call to
+``printf(var, 42)`` would be untouched. A call to ``printf("%d", 42)`` would
+become a call to ``__modular_printf`` with the same arguments, as would
+``printf("%f", 42.0)``. The latter would be accompanied with a strong
+relocation against the symbol ``__printf_float``, which would bring floating
+point support for ``printf`` into the link.
+
 The following aspects are currently supported:
 
 - ``float``: The call has a floating point argument