Update from_chars documentation with regard to whitespace (#1020)

Author: mrks

absl/strings/charconv.h

@@ -64,8 +64,9 @@ struct from_chars_result {
 // the result in `value`.
 //
 // The matching pattern format is almost the same as that of strtod(), except
-// that C locale is not respected, and an initial '+' character in the input
-// range will never be matched.
+// that (1) C locale is not respected, (2) an initial '+' character in the
+// input range will never be matched, and (3) leading whitespaces are not
+// ignored.
 //
 // If `fmt` is set, it must be one of the enumerator values of the chars_format.
 // (This is despite the fact that chars_format is a bitmask type.)  If set to