Describe max_width/ max_height, width_policy, pn.config.sizing_mode in size.md docs

[MarcSkovMadsen]

While helping BlueBit in https://discourse.holoviz.org/t/the-indicators-do-not-get-the-fixed-size-and-a-part-of-them-got-invisible-on-the-layout/5844 I discovered that

• max_width/ max_height is not described in Control Size
• pn.config.sizing_mode/ pn.extension(sizing_mode=...) is not described in Control Size
• width_policy/ height_policy is used but not described in Control Size
• pn.config.sizing_mode/ pn.extension(sizing_mode=...)

I could not using search or my knowledge of the docs find other relevant places where they where described.