system-variables: add tidb_paging_size_bytes (internal use)

[JmPotato]

What is changed, added or deleted? (Required)

新增系统变量 tidb_paging_size_bytes 的文档说明。

该变量由 pingcap/tidb#68091 引入，用于在 coprocessor 分页协议中，按字节数为单个 paging 响应设置上限（在按行数分页 tidb_max_paging_size 之外，额外提供按字节分页的能力）。默认值为 0（关闭），仅在开启资源管控时生效，供 PD 资源管控做 RU 预扣费使用。

该变量为 TiDB 内部使用的变量，因此文档中已明确标注 不推荐修改该变量的值。

字段（作用域、是否持久化、SET_VAR Hint、类型、默认值、范围、单位）均取自源码定义（ScopeGlobal | ScopeSession、TypeUnsigned、默认 0、范围 [0, math.MaxInt64]）。

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)

What is the related PR or file link(s)?

• Other reference link(s):
  • pingcap/tidb PR: *: add tidb_paging_size_bytes and forward it to coprocessor RPC tidb#68091
  • kvproto PR: proto: add paging_size_bytes to coprocessor.Request kvproto#1448

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
Once this PR has been reviewed and has the lgtm label, please assign breezewish for approval. For more information see the Code Review Process.
Please ensure that each of them provides their approval before proceeding.

The full list of commands accepted by this bot can be found here.

Details

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment