mm/dynamic_pool: add Document for dynamic hugetlb feature

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I8S9BY


CVE: NA

--------------------------------

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>