Doc comment maintenance

Fixed doc comments issues:

* Fixed mistyped param doc comments (`/**<` is OK, `/** <` is not).

* Put special characters (e.g., pipe, backslash, etc.) in quotes, as they can
  confuse doxygen and it will print lots of various warnings. For the sake of
  completeness and consistent style, also quote some special characters in
  re-bytecode.h

* Added missing `@{`s, removed extra `@}`s.

* Turned `/*` comments to `/**<` doc comments.

Ensured same style for doc groups everywhere:

* Where `\addtogroup`, `@{`, and `@}` doxygen commands are used, the order to be
  followed is: license, `#ifndef` guards (in headers), includes, `\addtogroup`
  and `@{`, main code content, `@}`, `#endif` guards (in headers).

* Multiple `\addtogroup`s or multiple `@}`s should be in the same doc comment.

* First `\addtogroup` should be on the very first line of a doc comment, i.e.,
  `/** \addtogroup`.

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

jerry-core/ecma/base/ecma-helpers-conversion.c

@@ -14,20 +14,18 @@
  * limitations under the License.
  */
 
-/** \addtogroup ecma ECMA
- * @{
- *
- * \addtogroup ecmahelpers Helpers for operations with ECMA data types
- * @{
- */
-
 #include "ecma-globals.h"
 #include "ecma-helpers.h"
 #include "jrt-libc-includes.h"
 #include "lit-char-helpers.h"
 #include "lit-magic-strings.h"
 
-/*
+/** \addtogroup ecma ECMA
+ * @{
+ *
+ * \addtogroup ecmahelpers Helpers for operations with ECMA data types
+ * @{
+ *
  * \addtogroup ecmahelpersbigintegers Helpers for operations intermediate 128-bit integers
  * @{
  */