[glibc] manual: Update memchr description [BZ #19406]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=368438d8b6e28b034dbf4722290ae4b3989bac57

commit 368438d8b6e28b034dbf4722290ae4b3989bac57
Author: Wilco Dijkstra <[email protected]>
Date:   Mon Jun 15 16:11:55 2026 +0000

    manual: Update memchr description [BZ #19406]
    
    Update memchr description to indicate it cannot overread and the size
    may be larger than the object passed.  This is required since C11.
    
    Suggested-by: Paul Eggert <[email protected]>
    Reviewed-by: Paul Eggert <[email protected]>

Diff:
---
 manual/string.texi | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/manual/string.texi b/manual/string.texi
index 24004f1f5c..b7a9ba0d46 100644
--- a/manual/string.texi
+++ b/manual/string.texi
@@ -1803,6 +1803,12 @@ to an @code{unsigned char}) in the initial @var{size} bytes of the
 object beginning at @var{block}.  The return value is a pointer to the
 located byte, or a null pointer if no match was found.
 
+This function behaves as if it reads the bytes sequentially and stops at
+the first match, which means that if the object contains a match the
+behavior is defined even when the object size is less than @var{size}.
+This behavior guarantee is unusual, and is not shared by related
+functions like @code{memcmp} and @code{wmemchr}.
+
 In ISO C23 and later, this function is qualifier-generic:
 that is, it is also implemented as a function-like macro,
 and when the macro is used and @var{block} has a type