PoV mode + complex benchmarking documentation

[kianenigma]

after some years of break, I started working on benchmarking again recently.

The basics work, and are well documented. A lot of advance features are undocumented and nuanced.

• There is no mention of pov_mod syntax in https://paritytech.github.io/polkadot-sdk/master/frame_benchmarking/v2/index.html
• There is no mention of it in https://paritytech.github.io/polkadot-sdk/master/polkadot_sdk_docs/reference_docs/frame_benchmarking_weight/index.html
• Mention the nuance of using state-version: 1
• Mention how to interpret removals as writes

[gui1117]

Not exactly related but PoV cost is asynchronously metered (We can query at any time the PoV weight), only ref time needs PVM.
So should we start documenting that PoV benchmark can be implemented in a more lenient manner? And saying reasonable over estimation is ok as weight is accurately reclaimed?

That said there is still some current inaccuracy in the weight reclaim: #6020