feat: Handle getters and setters in documentation template assist

[jonas-schievink]

The assist can now turn this:

pub struct S;
impl S {
    pub fn data_mut$0(&mut self) -> &mut [u8] { &mut [] }
}

into

pub struct S;
impl S {
    /// Returns a mutable reference to the data.
    ///
    /// # Examples
    ///
    /// ```
    /// use test::S;
    ///
    /// let mut s = ;
    /// assert_eq!(s.data_mut(), );
    /// assert_eq!(s, );
    /// ```
    pub fn data_mut(&mut self) -> &mut [u8] { &mut [] }
}

And similarly for by-value or immutable getters, and for setters. Previously the intro line would be empty.

This PR also removes the documentation generation function from the "Generate getter/setter" assist, since that is better handled by applying the 2 assists in sequence. cc #12273

[jonas-schievink]

@bors r+

[bors]

📌 Commit f1b6e45 has been approved by jonas-schievink

[bors]

⌛ Testing commit f1b6e45 with merge da503b6...

[bors]

☀️ Test successful - checks-actions
Approved by: jonas-schievink
Pushing da503b6 to master...

[jonas-schievink]

Oh I forgot generate_setter is in a different file