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

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.