Add documentation for ck_assert_ptr_null and ck_assert_ptr_nonnull ma…

…cros

doc/check.texi

@@ -1217,6 +1217,17 @@ would pass.
 @item ck_assert_ptr_eq
 @itemx ck_assert_ptr_ne
 
+Compares a pointers against null and displays predefined message with
+condition and value of the input parameter on failure.
+@code{@item ck_assert_ptr_null} checks that pointer is equal to NULL and
+@code{@item ck_assert_ptr_nonnull} checks that pointer is not equal to NULL.
+@code{@item ck_assert_ptr_nonnull} is highly recommended to use in situations
+when a function call can return NULL as error indication (like functions
+that use malloc, calloc, strdup, mmap, etc).
+
+@item ck_assert_ptr_null
+@itemx ck_assert_ptr_nonnull
+
 Compares two pointers and displays predefined message with
 condition and values of both input parameters on failure.  The operator
 used for comparison is different for each function and is indicated by