For integers we're clear where the base prefix is inserted (which is important, because it's between a minus sign and the digits, not right at the start):

For integral types, the alternate form inserts the base prefix (if any) specified in Table 69 into the output after the sign character (possibly space) if there is one, or before the output of to_chars otherwise

For pointers, Table 73 just says "with the prefix 0x added to the output". Obviously, a prefix should be at the start (and there's no minus sign to worry about) but it might be better to be explicit, especially as the text about the prefix comes after saying we use to_chars.

Tim suggested something like "inserted before the output of to_chars."