fs: add missing documentation to simple_xattr functions

v2: add function documentation instead of adding a separate file under
    Documentation/

tj: Updated comment a bit and rolled in Randy's suggestions.

Cc: Li Zefan <lizefan@huawei.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Hillf Danton <dhillf@gmail.com>
Cc: Lennart Poettering <lpoetter@redhat.com>
Cc: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: NAristeu Rozanski <aris@redhat.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

fs/xattr.c

@@ -892,8 +892,19 @@ static int __simple_xattr_set(struct simple_xattrs *xattrs, const char *name,
 
 }
 
-/*
- * xattr SET operation for in-memory/pseudo filesystems
+/**
+ * simple_xattr_set - xattr SET operation for in-memory/pseudo filesystems
+ * @xattrs: target simple_xattr list
+ * @name: name of the new extended attribute
+ * @value: value of the new xattr. If %NULL, will remove the attribute
+ * @size: size of the new xattr
+ * @flags: %XATTR_{CREATE|REPLACE}
+ *
+ * %XATTR_CREATE is set, the xattr shouldn't exist already; otherwise fails
+ * with -EEXIST.  If %XATTR_REPLACE is set, the xattr should exist;
+ * otherwise, fails with -ENODATA.
+ *
+ * Returns 0 on success, -errno on failure.
  */
 int simple_xattr_set(struct simple_xattrs *xattrs, const char *name,
 		     const void *value, size_t size, int flags)
@@ -950,6 +961,9 @@ ssize_t simple_xattr_list(struct simple_xattrs *xattrs, char *buffer,
 	return used;
 }
 
+/*
+ * Adds an extended attribute to the list
+ */
 void simple_xattr_list_add(struct simple_xattrs *xattrs,
 			   struct simple_xattr *new_xattr)
 {