doxygen: enable AUTOBRIEF, drop \brief

Especially for short descriptions, it is annoying to have to type \brief for
every single API doc.

Drop all \brief and enable the AUTOBRIEF feature of doxygen, which always takes
the first sentence of an API doc as the brief description.

Change-Id: I11a8a821b065a128108641a2a63fb5a2b1916e87
diff --git a/Doxyfile.gb.in b/Doxyfile.gb.in
index 36b4f2b..e6b7826 100644
--- a/Doxyfile.gb.in
+++ b/Doxyfile.gb.in
@@ -156,7 +156,7 @@
 # interpret the first line (until the first dot) of a Qt-style
 # comment as the brief description. If set to NO, the comments
 # will behave just like regular Qt-style comments (thus requiring
-# an explicit \brief command for a brief description.)
+# an explicit command for a brief description.)
 
 QT_AUTOBRIEF           = YES