Add notes for remap's return when istart and istop are the same

Co-Authored-By: Rémi Verschelde <rverschelde@gmail.com>
Co-Authored-By: kleonc <9283098+kleonc@users.noreply.github.com>

doc/classes/@GlobalScope.xml

@@ -1097,6 +1097,7 @@
 				remap(75, 0, 100, -1, 1) # Returns 0.5
 				[/codeblock]
 				For complex use cases where multiple ranges are needed, consider using [Curve] or [Gradient] instead.
+				[b]Note:[/b] If [code]istart == istop[/code], the return value is undefined (most likely NaN, INF, or -INF).
 			</description>
 		</method>
 		<method name="rid_allocate_id">