Documentation: debugfs: Document debugfs helper for unsigned long values

When debugfs_create_ulong() was added, it was not documented.

Fixes: c23fe831 ("debugfs: Add debugfs_create_ulong()")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Link: https://lore.kernel.org/r/20191021150645.32440-1-geert+renesas@glider.beSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/filesystems/debugfs.txt

@@ -93,8 +93,8 @@ the following functions can be used instead:
 
 These functions are useful as long as the developer knows the size of the
 value to be exported.  Some types can have different widths on different
-architectures, though, complicating the situation somewhat.  There is a
-function meant to help out in one special case:
+architectures, though, complicating the situation somewhat.  There are
+functions meant to help out in such special cases:
 
     void debugfs_create_size_t(const char *name, umode_t mode,
 			       struct dentry *parent, size_t *value);
@@ -102,6 +102,12 @@ function meant to help out in one special case:
 As might be expected, this function will create a debugfs file to represent
 a variable of type size_t.
 
+Similarly, there is a helper for variables of type unsigned long:
+
+    struct dentry *debugfs_create_ulong(const char *name, umode_t mode,
+					struct dentry *parent,
+					unsigned long *value);
+
 Boolean values can be placed in debugfs with:
 
     struct dentry *debugfs_create_bool(const char *name, umode_t mode,