Fix VariantWriter overflow on 64-bit int

Integers in Godot are signed 64-bit ints (int64_t), but var2str used
int behind the scenes and would thus overflow after 2^31.

Also properly documented the actual bounds of int and the behaviour
when overflowing them.

doc/classes/@GDScript.xml

@@ -811,7 +811,7 @@
 			<description>
 				Random range, any floating point value between [code]from[/code] and [code]to[/code].
 				[codeblock]
-				prints(rand_range(0, 1), rand_range(0, 1)) # prints 0.135591 0.405263
+				prints(rand_range(0, 1), rand_range(0, 1)) # prints e.g. 0.135591 0.405263
 				[/codeblock]
 			</description>
 		</method>
@@ -830,7 +830,7 @@
 			<description>
 				Returns a random floating point value on the interval [code][0, 1][/code].
 				[codeblock]
-				randf() # returns 0.375671
+				randf() # returns e.g. 0.375671
 				[/codeblock]
 			</description>
 		</method>
@@ -838,11 +838,12 @@
 			<return type="int">
 			</return>
 			<description>
-				Returns a random 32 bit integer. Use remainder to obtain a random value in the interval [code][0, N][/code] (where N is smaller than 2^32 -1).
+				Returns a random unsigned 32 bit integer. Use remainder to obtain a random value in the interval [code][0, N][/code] (where N is smaller than 2^32 -1).
 				[codeblock]
-				randi() % 20      # returns random number between 0 and 19
-				randi() % 100     # returns random number between 0 and 99
-				randi() % 100 + 1 # returns random number between 1 and 100
+				randi()           # returns random integer between 0 and 2^32 - 1
+				randi() % 20      # returns random integer between 0 and 19
+				randi() % 100     # returns random integer between 0 and 99
+				randi() % 100 + 1 # returns random integer between 1 and 100
 				[/codeblock]
 			</description>
 		</method>