PR #3414 added support for disabling the new PEP660 editable install
hooks. However the documentation and changelog mentions didn't
match the implementation.

Before:
- The implementation used: `SETUPTOOLS_ENABLE_FEATURES`
- The changelog said to use: `SETUPTOOLS_ENABLE_FEATURE`
  (notice the missing "S")
- The docs said to use: `SETUPTOOLS_USE_FEATURE`

This caused confusion in #3535, since the testcase there used the form
mentioned in the changelog, which doesn't do anything.

Now, the changelog and docs both say to use `SETUPTOOLS_ENABLE_FEATURES`.

The heading "How editable installations work?" either contains a typo (the question mark should be omitted), or else the question as written is missing a word (it should be "How **do** editable installations work?"). This commit removes the question mark.

`PATHONPATH` --> `PYTHONPATH`