[backport] correctly document toInt, fixes #2764 [ci skip] (#10176)

(cherry picked from commit 77166ba)

lib/system.nim

@@ -1772,14 +1772,16 @@ proc toBiggestFloat*(i: BiggestInt): BiggestFloat {.
 proc toInt*(f: float): int {.
   magic: "ToInt", noSideEffect, importc: "toInt".}
   ## converts a floating point number `f` into an ``int``. Conversion
-  ## rounds `f` if it does not contain an integer value. If the conversion
-  ## fails (because `f` is infinite for example), `ValueError` is raised.
+  ## rounds `f` if it does not contain an integer value.
+  ## Note that some floating point numbers (e.g. infinity) cannot be
+  ## accurately converted.
 
 proc toBiggestInt*(f: BiggestFloat): BiggestInt {.
   magic: "ToBiggestInt", noSideEffect, importc: "toBiggestInt".}
   ## converts a biggestfloat `f` into a ``biggestint``. Conversion
-  ## rounds `f` if it does not contain an integer value. If the conversion
-  ## fails (because `f` is infinite for example), `ValueError` is raised.
+  ## rounds `f` if it does not contain an integer value.
+  ## Note that some floating point numbers (e.g. infinity) cannot be
+  ## accurately converted.
 
 proc addQuitProc*(QuitProc: proc() {.noconv.}) {.
   importc: "atexit", header: "<stdlib.h>".}