doc: printf %b: clarify octal processing

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