Add operational mode for jerry_gc API call. (#2385)

JerryScript-DCO-1.0-Signed-off-by: Zoltan Herczeg zherczeg.u-szeged@partner.samsung.com

docs/02.API-REFERENCE.md

@@ -68,6 +68,18 @@ Option bits for [jerry_parse](#jerry_parse) and
  - JERRY_PARSE_NO_OPTS - no options passed
  - JERRY_PARSE_STRICT_MODE - enable strict mode
 
+## jerry_gc_mode_t
+
+Set garbage collection operational mode
+
+ - JERRY_GC_SEVERITY_LOW - free unused objects
+ - JERRY_GC_SEVERITY_HIGH - free as much memory as possible
+
+The difference between `JERRY_GC_SEVERITY_LOW` and `JERRY_GC_SEVERITY_HIGH`
+is that the former keeps memory allocated for performance improvements such
+as property hash tables for large objects. The latter frees all possible
+memory blocks but the performance may drop after the garbage collection.
+
 ## jerry_generate_snapshot_opts_t
 
 Flags for [jerry_generate_snapshot](#jerry_generate_snapshot) and
@@ -705,13 +717,30 @@ Performs garbage collection.
 
 ```c
 void
-jerry_gc (void);
+jerry_gc (jerry_gc_mode_t mode);
 ```
 
+- `mode` - operational mode, see [jerry_gc_mode_t](#jerry_gc_mode_t)
+
 **Example**
 
+[doctest]: # ()
+
 ```c
-jerry_gc ();
+#include "jerryscript.h"
+
+int
+main (void)
+{
+  jerry_init (JERRY_INIT_EMPTY);
+
+  jerry_value_t object_value = jerry_create_object ();
+  jerry_release_value (object_value);
+
+  jerry_gc (JERRY_GC_SEVERITY_LOW);
+
+  jerry_cleanup ();
+}
 ```
 
 **See also**
@@ -1601,7 +1630,7 @@ jerry_value_is_undefined (const jerry_value_t value)
 Returns the JavaScript type
 for a given value as a [jerry_type_t](#jerry_type_t) enum value.
 
-This is a similar operation as the 'typeof' operator
+This is a similar operation to the 'typeof' operator
 in the standard with an exception that the 'null'
 value has its own enum value.