Fix FILE_PATTERNS of Doxyfile and some of the issues it was hiding (#2446)

`FILE_PATTERNS` is a space-separated list, in contrary to what is
suggested by its documentation. The pattern `*.h, *.c` does not
match header files but files with `.h,` extension only. Rewriting
the current comma-separated pattern makes Doxygen actually process
header files. However, it also reveals several hitherto hidden
issues (mostly missing documentation) in the code. This patch fixes
some of these documentation problems (and lists the files with
outstanding issues in a 'backlog').

JerryScript-DCO-1.0-Signed-off-by: Akos Kiss akiss@inf.u-szeged.hu

jerry-core/vm/vm-defines.h

@@ -26,9 +26,13 @@
  */
 
 /**
- * Helpers for updating uint16_t values.
+ * Helper for += on uint16_t values.
  */
 #define VM_PLUS_EQUAL_U16(base, value) (base) = (uint16_t) ((base) + (value))
+
+/**
+ * Helper for -= on uint16_t values.
+ */
 #define VM_MINUS_EQUAL_U16(base, value) (base) = (uint16_t) ((base) - (value))
 
 /**