[Documentation] 补充markpoint textStyle 文档

[skie1997]

Documentation Title or Section

功能pr: https://github.com/VisActor/VChart/pull/3979/files

Issue Description or Suggestion

补充文档

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Documentation Title or Section

Function pr: https://github.com/VisActor/VChart/pull/3979/files

Issue Description or Suggestion

Supplementary documentation