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

jerry-core/api/jerry-snapshot.c

@@ -31,6 +31,10 @@
 #include "lit-char-helpers.h"
 #include "re-compiler.h"
 
+/** \addtogroup jerrysnapshot Jerry snapshot operations
+ * @{
+ */
+
 #if JERRY_SNAPSHOT_SAVE || JERRY_SNAPSHOT_EXEC
 
 /**
@@ -88,9 +92,6 @@ typedef struct
   bool class_found;
 } snapshot_globals_t;
 
-/** \addtogroup jerrysnapshot Jerry snapshot operations
- * @{
- */
 
 /**
  * Write data into the specified buffer.
@@ -1045,10 +1046,6 @@ jerry_exec_snapshot (const uint32_t *snapshot_p, /**< snapshot */
 #endif /* JERRY_SNAPSHOT_EXEC */
 } /* jerry_exec_snapshot */
 
-/**
- * @}
- */
-
 #if JERRY_SNAPSHOT_SAVE
 
 /**
@@ -1716,3 +1713,7 @@ jerry_get_literals_from_snapshot (const uint32_t *snapshot_p, /**< input snapsho
   return 0;
 #endif /* JERRY_SNAPSHOT_SAVE */
 } /* jerry_get_literals_from_snapshot */
+
+/**
+ * @}
+ */

jerry-core/api/jerryscript.c

@@ -115,6 +115,8 @@ JERRY_STATIC_ASSERT (((NUMBER_ARITHMETIC_SUBTRACTION + ECMA_NUMBER_ARITHMETIC_OP
  *         The API could not be invoked in the following cases:
  *           - before jerry_init and after jerry_cleanup
  *           - between enter to and return from a native free callback
+ *
+ * @return void
  */
 static inline void JERRY_ATTR_ALWAYS_INLINE
 jerry_assert_api_enabled (void)
@@ -124,6 +126,8 @@ jerry_assert_api_enabled (void)
 
 /**
  * Turn on API availability
+ *
+ * @return void
  */
 static inline void JERRY_ATTR_ALWAYS_INLINE
 jerry_api_enable (void)
@@ -135,6 +139,8 @@ jerry_api_enable (void)
 
 /**
  * Turn off API availability
+ *
+ * @return void
  */
 static inline void JERRY_ATTR_ALWAYS_INLINE
 jerry_api_disable (void)