manual: clarify errno value on success [BZ #22615]

The current glibc manual is ambiguous about the errno value on success
and suggests that it is left unchanged. Some functions might and
sometimes do change the errno value, however they never set it to 0.

This patch from Zack Weinberg clarifies this section of the manual.

Changelog:
	[BZ #22615]
	* manual/errno.texi (Checking for Errors): Explicitly say that errno
	might be set on success.

1 files changed, 6 insertions, 0 deletions

diff --git a/ChangeLog b/ChangeLog
index 02f491b6b0..f87bc14690 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+2017-12-29  Zack Weinberg  <zackw@panix.com>
+
+	[BZ #22615]
+	* manual/errno.texi (Checking for Errors): Explicitly say that errno
+	might be set on success.
+
 2017-12-29  Aurelien Jarno  <aurelien@aurel32.net>
 
 	[BZ #22611]