Update Doxygen guide to improve autobrief processing

Unlike docstring blocks starting with '\**', Doxygen does not
automatically process brief documentations that start with '\*!'.

See https://www.doxygen.nl/manual/docblocks.html

CONTRIBUTING.md

@@ -67,7 +67,7 @@
 
 * All classes, member variables, and methods should have Doxygen-style comments
   (for example, comment lines starting with `//!` or comment blocks starting with
-  `/*!`; do not use `///` or `/**` in new code)
+  `/**`; do not use `///` or `/*!` in new code)
 * Doxygen-style groupings should bracket code using `//! @{` and `//! @}`
 * Doxygen commands should use the `@` prefix instead of `\` in order to better
   differentiate from LaTeX input.