Explain `'w` and `'s` lifetimes better in documentation

[harudagondi]

How can Bevy's documentation be improved?

There are a lot of types in bevy_ecs that has the 'w and 's lifetimes:

• EventReader
• EventWriter
• QueryIter
• QueryManyIter
• QueryCombinationIter
• SystemParamItem
• ParamSet
• Commands
• EntityCommands
• and more...

The best explanation found for these lifetimes are in SystemParam

Solutions I could think of right now:

1. Rename 'w and 's to 'world and 'state. This is not ideal since this will make the code more verbose.
2. Link users to the SystemParam documentation. There are a lot of types to document and imo would add more noise to their documentation.
3. Put the documentation in the README, perhaps under a new "Advanced topics" sections.

[alice-i-cecile]

These belong in the module level docs, and should be linked to wherever those lifetimes are used :)

[CGMossa]

I'd vote for calling them 'world and 'state also..