Restore extraction of brief descriptions

As of Doxygen 1.8.19 (specifically,
doxygen/doxygen@327423e),
the first line of a comment is no longer automatically extracted as a
brief description unless we enable the JAVADOC_AUTOBRIEF setting. See
https://www.doxygen.nl/manual/config.html#cfg_javadoc_autobrief:

> If the `JAVADOC_AUTOBRIEF` tag is set to `YES` then doxygen will
> interpret the first line (until the first dot) of a Javadoc-style
> comment as the brief description. If set to `NO`, the Javadoc-style
> will behave just like regular Qt-style comments (thus requiring an
> explicit @brief command for a brief description.)

Fixes #1563.

docs/Doxyfile.in

@@ -52,6 +52,9 @@ EXTRACT_ALL            = NO
 ALWAYS_DETAILED_SEC    = NO
 #REPEAT_BRIEF           = NO
 
+# Needed to automatically extract implicit brief descriptions in recent versions of Doxygen
+JAVADOC_AUTOBRIEF      = YES
+
 # Need these next options to ensure that functions with modifiers do not confuse the Doxygen parser.
 # And any further function modifiers here.
 MACRO_EXPANSION        = YES