[PATCH] manual: Document optind zero set behaviour (BZ#23157)

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

POSIX [1] does not explicit state the expected way to rescans the same
vector more than once.  FreeBSD [2], for instance, exports a non-standard
variable 'optreset' which must be set to '1' prior the second and each
additional call to 'getopt'.  GLIBC in turn requires the program to
reset 'optind' to 0 instead (and POSIX states the behavior is unspecified).

Unfortunately this is not documented on the manual, only on man-pages [3]
(on NOTES).  This patch adds an explanation of this behavior on manual.

	* manual/getopt.texi: Document optind zero set behaviour.

[1] http://pubs.opengroup.org/onlinepubs/9699919799/
[2] https://www.freebsd.org/cgi/man.cgi?getopt(3)
[3] http://man7.org/linux/man-pages/man3/getopt.3.html
---
 ChangeLog          | 4 ++++
 manual/getopt.texi | 6 ++++++
 2 files changed, 10 insertions(+)

diff --git a/manual/getopt.texi b/manual/getopt.texi
index 5485fc4..a4f6366 100644
--- a/manual/getopt.texi
+++ b/manual/getopt.texi
@@ -45,6 +45,12 @@ of the @var{argv} array to be processed.  Once @code{getopt} has found
 all of the option arguments, you can use this variable to determine
 where the remaining non-option arguments begin.  The initial value of
 this variable is @code{1}.
+
+Resetting the variable value to @code{0} forces the invocation of an
+internal initialization routine and it is used mainly when a program
+wants to rescan the same vector more than once.  It also should be used
+to scan multiple argument vectors or if @code{POSIXLY_CORRECT} is changed
+between scans.
 @end deftypevar
 
 @deftypevar {char *} optarg
-- 
2.7.4