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

[Wilco Dijkstra]

Update memchr description to indicate it cannot overread and the
size input may be larger than the object passed.  This is required
since C11. Fixes BZ 19406.

OK for commit?

---

diff --git a/manual/string.texi b/manual/string.texi
index 24004f1f5cc60b70bafde0794351734a4eb1ce2b..793fbd230163d09390232ede67225f68b6ea6e4a 100644
--- a/manual/string.texi
+++ b/manual/string.texi
@@ -1803,6 +1803,10 @@ 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 each byte is read sequentially and stops as
+soon as the first match is found.  @var{size} may be larger than the object
+starting at @var{block} if there is a match within the object.
+
 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