text/template: explain that integer constants can overflow

This behavior is the same as in Go: constants can be coerced to int
and whether overflow occurs depends on how big an int is, but
this surprises people sometimes, so document it again here.

Fixes #25833.

Change-Id: I557995f1a1e8e871b21004953923d16f36cb9037
Reviewed-on: https://go-review.googlesource.com/c/141378Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/text/template/doc.go

@@ -142,7 +142,9 @@ An argument is a simple value, denoted by one of the following.
 
 	- A boolean, string, character, integer, floating-point, imaginary
 	  or complex constant in Go syntax. These behave like Go's untyped
-	  constants.
+	  constants. Note that, as in Go, whether a large integer constant
+	  overflows when assigned or passed to a function can depend on whether
+	  the host machine's ints are 32 or 64 bits.
 	- The keyword nil, representing an untyped Go nil.
 	- The character '.' (period):
 		.