[format.string] Avoid duplicating the specification of '#' for integers.

Fix the specification for '#' being different for octal integers in the
two places it's specified.

Author: zygoloid

source/utilities.tex

@@ -19892,27 +19892,20 @@
 This option is only valid for arithmetic types other than
 \tcode{charT} and \tcode{bool}
 or when an integer presentation type is specified.
-For integers,
-when binary or hexadecimal output is used,
-this option adds the respective prefix
-\tcode{0b} (\tcode{0B}) or
-\tcode{0x} (\tcode{0X})
+For integral types,
+the alternate form adds the
+base prefix (if any) specified in \tref{format.type.int}
 to the output value.
-For nonzero integers,
-when octal output is used,
-this option adds the prefix \tcode{0}
-to the output value.
-Whether the prefix is lower-case or upper-case
-is determined by the case of the type format specifier.
-For floating-point numbers
+For floating-point types,
 the alternate form causes the result of the conversion
 to always contain a decimal-point character,
 even if no digits follow it.
 % FIXME: This is a weird place for this part of the spec to appear.
 Normally, a decimal-point character appears in the result of these
 conversions only if a digit follows it.
 In addition, for \tcode{g} and \tcode{G} conversions,
-% FIXME: Are they normally?
+% FIXME: Are they normally? What does this even mean? Reach into to_chars and
+% alter its behavior?
 trailing zeros are not removed from the result.
 
 \pnum
@@ -20000,13 +19993,14 @@
 \lhdr{Type} & \rhdr{Meaning} \\ \rowsep
 \tcode{b} &
 \tcode{to_chars(first, last, value, 2)};
-% FIXME: This duplicates things we said earlier.
-using the \tcode{\#} option with this type adds the prefix \tcode{0b} to the output.
+\indextext{base prefix}%
+the base prefix is \tcode{0b}.
 \\ \rowsep
 %
 \tcode{B} &
 The same as \tcode{b}, except that
-the \tcode{\#} option adds the prefix \tcode{0B} to the output.
+\indextext{base prefix}%
+the base prefix is \tcode{0B}.
 \\ \rowsep
 %
 \tcode{c} &
@@ -20021,18 +20015,21 @@
 %
 \tcode{o} &
 \tcode{to_chars(first, last, value, 8)};
-using the \tcode{\#} option with this type adds the prefix \tcode{0} to the output.
+\indextext{base prefix}%
+the base prefix is \tcode{0} if \tcode{value} is nonzero and is empty otherwise.
 \\ \rowsep
 %
 \tcode{x} &
 \tcode{to_chars(first, last, value, 16)};
-using the \tcode{\#} option with this type adds the prefix \tcode{0x} to the output.
+\indextext{base prefix}%
+the base prefix is \tcode{0x}.
 \\ \rowsep
 %
 \tcode{X} &
 The same as \tcode{x}, except that
 it uses uppercase letters for digits above 9 and
-the \tcode{\#} option adds the prefix \tcode{0X} to the output.
+\indextext{base prefix}%
+the base prefix is \tcode{0X}.
 \\ \rowsep
 %
 \tcode{n} &