doc: printf %b: clarify octal processing

* src/printf.c: Remove redundant comment.
State explicitly that the leading 0 is the exception
from normal escape processing.  Remove a full stop for consistency.
* doc/coreutils.texi (printf invocation): Add a reference
to C99 string escapes since these are not mentioned
in the referenced glibc printf info. Also explicitly state
the leading 0 exception.  Also use NNN rather than OOO
to be consistent with the --help documentation.
Also remove and extraneous '\' and fix grammar in the info
regarding the ninth bit.
Addresses https://bugs.gnu.org/72657

doc/coreutils.texi

@@ -13401,10 +13401,12 @@ printf @var{format} [@var{argument}]@dots{}
 
 @command{printf} prints the @var{format} string, interpreting @samp{%}
 directives and @samp{\} escapes to format numeric and string arguments
-in a way that is mostly similar to the C @samp{printf} function.
+in a way that is mostly similar to the C @samp{printf} function,
+and C language escape sequence processing.
 @xref{Output Conversion Syntax,, @command{printf} format directives,
 libc, The GNU C Library Reference Manual}, for details.
-The differences are listed below.
+See also @uref{https://en.cppreference.com/w/c/language/escape,
+C99 string escapes:}.  The differences are listed below.
 
 @mayConflictWithShellBuiltIn{printf}
 
@@ -13437,9 +13439,9 @@ one.
 @kindex %b
 An additional directive @samp{%b}, prints its
 argument string with @samp{\} escapes interpreted in the same way as in
-the @var{format} string, except that octal escapes are of the form
-@samp{\0@var{ooo}} where @var{ooo} is 0 to 3 octal digits.  If
-@samp{\@var{ooo}} is nine-bit value, ignore the ninth bit.
+the @var{format} string, except that octal escapes should have a leading
+@samp{0} like @samp{\0@var{nnn}}.
+If @samp{@var{nnn}} is a nine-bit value, ignore the ninth bit.
 If a precision is also given, it limits the number of bytes printed
 from the converted string.