mm/dynamic_hugetlb: add Document for dynamic hugetlb feature

hulk inclusion
category: feature
bugzilla: 46904, https://gitee.com/openeuler/kernel/issues/I4QSHG


CVE: NA

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Add a Document to describe dynamic hugetlb feature, including the conflict
description, usage description and interface description.

Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>