f2fs: add missing documents of reserve_root/resuid/resgid

Add missing documents.

Fixes: 7e65be49 ("f2fs: add reserved blocks for root user")
Fixes: 7c2e5963 ("f2fs: add resgid and resuid to reserve root blocks")
Signed-off-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

Documentation/filesystems/f2fs.txt

@@ -157,6 +157,11 @@ noinline_data          Disable the inline data feature, inline data feature is
                        enabled by default.
 data_flush             Enable data flushing before checkpoint in order to
                        persist data of regular and symlink.
+reserve_root=%d        Support configuring reserved space which is used for
+                       allocation from a privileged user with specified uid or
+                       gid, unit: 4KB, the default limit is 0.2% of user blocks.
+resuid=%d              The user ID which may use the reserved blocks.
+resgid=%d              The group ID which may use the reserved blocks.
 fault_injection=%d     Enable fault injection in all supported types with
                        specified injection rate.
 fault_type=%d          Support configuring fault injection type, should be