Incomplete documentation for env_version

Describe the bug

https://www.sphinx-doc.org/en/master/extdev/index.html#extension-metadata documents env_version.

• It says:

  [...] env data structure if the extension stores any data to environment

  ... but it gives no indication how/why to do that.
  It could e.g. link to https://www.sphinx-doc.org/en/master/development/tutorials/todo.html

• It says:

  If not given, Sphinx considers the extension does not stores any data to environment.

  ... but it doesn't say if that includes a Domain (https://www.sphinx-doc.org/en/master/extdev/domainapi.html) defined by the extension. I assume that the domain is somehow stored in the environment, but I guess it doesn't need to be reflected in env_version because it tracks its own structure with the data_version attribute?

  • in the meantime it has been clarified that this is about actively storing stuff in the "env object", and not about more generally "in the environment", so I think this should be clear now.

• https://www.sphinx-doc.org/en/master/development/tutorials/todo.html shows an extension that stores data in the environment, but it does not return env_version in its setup() function!

  • potential fix: #12349

• https://www.sphinx-doc.org/en/master/development/tutorials/todo.html could mention that alternatively, a Domain could be used to store data in the environment. It could link to https://www.sphinx-doc.org/en/master/development/tutorials/recipe.html, which provides an example for that.

• https://www.sphinx-doc.org/en/master/development/tutorials/recipe.html does not mention data_version, which is of course not strictly necessary, but I think it might be a good idea to recommend specifying it, to make it easier to not forget bumping it once there are changes to the data at a later time.

  • potential fix: #12350

• This has nothing to do with env_version, but looking at the tutorials, I have seen that https://www.sphinx-doc.org/en/master/development/tutorials/autodoc_ext.html does not set parallel_read_safe but I guess there is no reason for this extension to disable parallelization, right?

  • this was fixed in #12288

• The docstring of Domain doesn't give any information whether it is recommended to use it to store arbitrary data (see https://github.com/sphinx-doc/sphinx/issues/12237#issuecomment-2047118111)

How to Reproduce

Look at https://www.sphinx-doc.org/en/master/extdev/index.html#extension-metadata

Read the extension metadata and domain API pages, then compare the TODO and recipe tutorials with sphinx/domains/init.py. Identify which unchecked documentation points remain after the referenced fixes, update the relevant documentation and Domain docstring, and verify the examples explain env_version and data_version consistently.