strconv: update documentation

Fixes #33750.
Updates #31197.

Change-Id: I26f63cef57e5f0eec85b84554c82f6d47b4f41a1
Reviewed-on: https://go-review.googlesource.com/c/go/+/191078Reviewed-by: Robert Griesemer <gri@golang.org>

src/strconv/atof.go

@@ -643,7 +643,7 @@ func atof64(s string) (f float64, err error) {
 // ParseFloat returns the nearest floating-point number rounded
 // using IEEE754 unbiased rounding.
 // (Parsing a hexadecimal floating-point value only rounds when
-// there are more bits in the hexadecimal representatiton than
+// there are more bits in the hexadecimal representation than
 // will fit in the mantissa.)
 //
 // The errors that ParseFloat returns have concrete type *NumError

src/strconv/atoi.go

@@ -154,7 +154,8 @@ func ParseUint(s string, base int, bitSize int) (uint64, error) {
 //
 // If base == 0, the base is implied by the string's prefix:
 // base 2 for "0b", base 8 for "0" or "0o", base 16 for "0x",
-// and base 10 otherwise.
+// and base 10 otherwise. Also, for base == 0 only, underscore
+// characters are permitted per the Go integer literal syntax.
 // If base is below 0, is 1, or is above 36, an error is returned.
 //
 // The bitSize argument specifies the integer type