Document BUF_strnlen

Reviewed-by: NMatt Caswell <matt@openssl.org>

doc/crypto/buffer.pod

@@ -32,6 +32,8 @@ standard C library equivalents
 
  size_t BUF_strlcat(char *dst, const char *src, size_t size);
 
+ size_t BUF_strnlen(const char *str, size_t maxlen);
+
 =head1 DESCRIPTION
 
 The buffer library handles simple character arrays. Buffers are used for
@@ -50,11 +52,11 @@ BUF_MEM_grow() changes the size of an already existing buffer to
 B<len>. Any data already in the buffer is preserved if it increases in
 size.
 
-BUF_strdup(), BUF_strndup(), BUF_memdup(), BUF_strlcpy() and
-BUF_strlcat() are equivalents of the standard C library functions. The
-dup() functions use OPENSSL_malloc() underneath and so should be used
-in preference to the standard library for memory leak checking or
-replacing the malloc() function.
+BUF_strdup(), BUF_strndup(), BUF_memdup(), BUF_strlcpy(),
+BUF_strlcat() and BUF_strnlen are equivalents of the standard C
+library functions. The dup() functions use OPENSSL_malloc() underneath
+and so should be used in preference to the standard library for memory
+leak checking or replacing the malloc() function.
 
 Memory allocated from these functions should be freed up using the
 OPENSSL_free() function.