Update doxygen and fix documentation (#5106)

Update Doxyfile to version 1.9.1

Re-enable doxygen CI checker

Fix some regular comments that should have been doc comments

Document void return types for some inline functions explicitly

Move start of some doxygen groups so they are included always, and not left
out of certain ifdefs

Ignore some doxygen warnings:
    Member (function) is not documented in headers
    Documented empty return type in headers
    Argument has multiple @param documentation sections

JerryScript-DCO-1.0-Signed-off-by: Máté Tokodi mate.tokodi@szteszoftver.hu

tools/check-doxygen.sh

@@ -14,7 +14,11 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-doxygen 2>&1 >/dev/null | head -n 1000 | tee doxygen.log
+doxygen 2>&1 >/dev/null \
+  | grep -Pv "^\/.+\.h:\d+: warning: Member .+ \(function\) of group \w+ is not documented.$" \
+  | grep -Pv "^\/.+\.h:\d+: warning: documented empty return type of \w+$" \
+  | grep -Pv "^\/.+\.h:\d+: warning: argument '.+' from the argument list of .+ has multiple @param documentation sections$" \
+  | head -n 1000 | tee doxygen.log
 if [ -s doxygen.log ]
 then
   EXIT=1