Add notes about overlapping copies.

Add notes to each of these that specifically state that results are
undefined if the strings overlap. In the case of memcpy, we document
the overlapping behavior on FreeBSD (pre-existing). For str*, it is
left unspecified, however, since the default (and x86) implementations
do not handle overlapping strings properly.

PR: 223653
Sponsored by: Netflix

lib/libc/string/memcpy.3

@@ -54,6 +54,11 @@ bytes from string
 .Fa src
 to string
 .Fa dst .
+If
+.Fa src
+and
+.Fa dst
+overlap, the results are not defined.
 .Sh RETURN VALUES
 The
 .Fn memcpy

lib/libc/string/strcat.3

@@ -62,6 +62,11 @@ then add a terminating
 The string
 .Fa s
 must have sufficient space to hold the result.
+If
+.Fa s
+and
+.Fa append
+overlap, the results are undefined.
 .Pp
 The
 .Fn strncat
@@ -72,6 +77,11 @@ characters from
 .Fa append ,
 and then adds a terminating
 .Ql \e0 .
+If
+.Fa s
+and
+.Fa append
+overlap, the results are undefined.
 .Sh RETURN VALUES
 The
 .Fn strcat

lib/libc/string/strcpy.3

@@ -63,6 +63,11 @@ to
 (including the terminating
 .Ql \e0
 character.)
+If
+.Fa src
+and
+.Fa dst
+overlap, the results are undefined.
 .Pp
 The
 .Fn stpncpy
@@ -89,6 +94,11 @@ Otherwise,
 is
 .Em not
 terminated.
+If
+.Fa src
+and
+.Fa dst
+overlap, the results are undefined.
 .Sh RETURN VALUES
 The
 .Fn strcpy