[Docs]add environment variables to environment_variables.md

[xiaolei373]

Motivation

添加环境变量的说明文档

Modifications

在environment_variables.md文档中新增了FD_CACHE_PROC_EXIT_TIMEOUT和FD_CACHE_PROC_EXIT_TIMEOUT说明

Usage or Command

export FD_CACHE_PROC_EXIT_TIMEOUT = 600
export FD_CACHE_PROC_ERROR_COUNT = 10

Accuracy Tests

Checklist

• Add at least a tag in the PR title.
  • Tag list: [[FDConfig],[APIServer],[Engine], [Scheduler], [PD Disaggregation], [Executor], [Graph Optimization], [Speculative Decoding], [RL], [Models], [Quantization], [Loader], [OP], [KVCache], [DataProcessor], [BugFix], [Docs], [CI], [Optimization], [Feature], [Benchmark], [Others], [XPU], [HPU], [GCU], [DCU], [Iluvatar], [Metax]]
  • You can add new tags based on the PR content, but the semantics must be clear.
• Format your code, run pre-commit before commit.
• Add unit tests. Please write the reason in this PR if no unit tests.
• Provide accuracy results.
• If the current PR is submitting to the release branch, make sure the PR has been submitted to the develop branch, then cherry-pick it to the release branch with the [Cherry-Pick] PR tag.

[paddle-bot]

Thanks for your contribution!

[EmmonsCurse]

LGTM