clarify wording on pure-functions overview to allow immutable hidden state, and add an example

[epic-64]

Updates the "pure functions" overview at https://docs.scala-lang.org/scala3/book/fp-pure-functions.html

• adds clarification that accessing immutable hidden state is still considered pure
• adds an example of this in form of a closure

[epic-64]

there are still some typos, sorry, marking it as draft until they are fixed