Tweak some structured suggestions to be more verbose and accurate

[estebank]

Addressing some issues I found while working on #127282.

error: this URL is not a hyperlink
  --> $DIR/auxiliary/include-str-bare-urls.md:1:11
   |
LL | HEADS UP! https://example.com MUST SHOW UP IN THE STDERR FILE!
   |           ^^^^^^^^^^^^^^^^^^^
   |
   = note: bare URLs are not automatically turned into clickable links
note: the lint level is defined here
  --> $DIR/include-str-bare-urls.rs:14:9
   |
LL | #![deny(rustdoc::bare_urls)]
   |         ^^^^^^^^^^^^^^^^^^
help: use an automatic link instead
   |
LL | HEADS UP! <https://example.com> MUST SHOW UP IN THE STDERR FILE!
   |           +                   +

error[E0384]: cannot assign twice to immutable variable `v`
  --> $DIR/assign-imm-local-twice.rs:7:5
   |
LL |     v = 1;
   |     ----- first assignment to `v`
LL |     println!("v={}", v);
LL |     v = 2;
   |     ^^^^^ cannot assign twice to immutable variable
   |
help: consider making this binding mutable
   |
LL |     let mut v: isize;
   |         +++

error[E0393]: the type parameter `Rhs` must be explicitly specified
  --> $DIR/issue-22560.rs:9:23
   |
LL | trait Sub<Rhs=Self> {
   | ------------------- type parameter `Rhs` must be specified for this
...
LL | type Test = dyn Add + Sub;
   |                       ^^^
   |
   = note: because of the default `Self` reference, type parameters must be specified on object types
help: set the type parameter to the desired type
   |
LL | type Test = dyn Add + Sub<Rhs>;
   |                          +++++

error[E0596]: cannot borrow `v` as mutable, as it is not declared as mutable
  --> $DIR/issue-33819.rs:4:34
   |
LL |         Some(ref v) => { let a = &mut v; },
   |                                  ^^^^^^ cannot borrow as mutable
   |
help: try removing `&mut` here
   |
LL -         Some(ref v) => { let a = &mut v; },
LL +         Some(ref v) => { let a = v; },
   |

help: remove the invocation before committing it to a version control system
   |
LL -     dbg!();
   |

error[E0308]: mismatched types
  --> $DIR/issue-39974.rs:1:21
   |
LL | const LENGTH: f64 = 2;
   |                     ^ expected `f64`, found integer
   |
help: use a float literal
   |
LL | const LENGTH: f64 = 2.0;
   |                      ++

error[E0529]: expected an array or slice, found `Vec<i32>`
  --> $DIR/match-ergonomics.rs:8:9
   |
LL |         [&v] => {},
   |         ^^^^ pattern cannot match with input type `Vec<i32>`
   |
help: consider slicing here
   |
LL |     match x[..] {
   |            ++++

error[E0609]: no field `0` on type `[u32; 1]`
  --> $DIR/parenthesized-deref-suggestion.rs:10:21
   |
LL |     (x as [u32; 1]).0;
   |                     ^ unknown field
   |
help: instead of using tuple indexing, use array indexing
   |
LL |     (x as [u32; 1])[0];
   |                    ~ +