Align expect messages with guidance

[pluiee]

Issue: #159751

• Changed 3 expect messages and some relevant comments in library/core/src/result.rs and library/core/src/primitive_docs.rs

• I intentionally left the below snippet unchanged in library/core/src/result.rs.
  The example is quite minimal and focuses on showing the panic output format, so I thought it was better to keep the message short and simple.

    /// # Panics
    ///
    /// Panics if the value is an [`Err`], with a panic message including the
    /// passed message, and the content of the [`Err`].
    ///
    ///
    /// # Examples
    ///
    /// ```should_panic
    /// let x: Result<u32, &str> = Err("emergency failure");
    /// x.expect("Testing expect"); // panics with `Testing expect: emergency failure`
    /// ```

r? @fee1-dead

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[mejrs]

This is far too specific, you can fail a write for other reasons.

View changes since the review

[mejrs]

This line is long enough that the example will get a horizontal scrollbar.
Can you make it shorter?

View changes since the review

[rustbot]

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