Clarify `String.get_slice` behavior

Clarify that the function returns the whole string if there is no
instances of the delimiter in the string.

doc/classes/String.xml

@@ -290,7 +290,7 @@
 			<param index="0" name="delimiter" type="String" />
 			<param index="1" name="slice" type="int" />
 			<description>
-				Splits the string using a [param delimiter] and returns the substring at index [param slice]. Returns an empty string if the [param slice] does not exist.
+				Splits the string using a [param delimiter] and returns the substring at index [param slice]. Returns the original string if [param delimiter] does not occur in the string. Returns an empty string if the [param slice] does not exist.
 				This is faster than [method split], if you only need one substring.
 				[b]Example:[/b]
 				[codeblock]