Fix warnings when running `cargo doc --document-private-items`

Fix warnings when running `cargo doc --document-private-items`

These were the warnings previously:

```
warning: could not parse code block as Rust code
   --> crates/stdx/src/lib.rs:137:9
    |
137 |   ///     ∀ x in slice[..idx]:  pred(x)
    |  _________^
138 | | ///  && ∀ x in slice[idx..]: !pred(x)
    | |____^
    |
    = note: error from rustc: unknown start of token: \u{2200}

warning: 1 warning emitted

warning: unresolved link to `package`
   --> crates/base_db/src/input.rs:181:15
    |
181 |     /// it's [package].name, can be different for other project types or even
    |               ^^^^^^^ no item named `package` in scope
    |
    = note: `#[warn(broken_intra_doc_links)]` on by default
    = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`

warning: unresolved link to `package`
   --> crates/base_db/src/input.rs:181:15
    |
181 |     /// it's [package].name, can be different for other project types or even
    |               ^^^^^^^ no item named `package` in scope
    |
    = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`

warning: 2 warnings emitted

warning: unresolved link to `package`
   --> crates/base_db/src/input.rs:181:15
    |
181 |     /// it's [package].name, can be different for other project types or even
    |               ^^^^^^^ no item named `package` in scope
    |
    = note: `#[warn(broken_intra_doc_links)]` on by default
    = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`

warning: unresolved link to `package`
   --> crates/base_db/src/input.rs:181:15
    |
181 |     /// it's [package].name, can be different for other project types or even
    |               ^^^^^^^ no item named `package` in scope
    |
    = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`

warning: 2 warnings emitted
```

This does *not* fix the following warning, because it is actually rust
code and rustdoc is being over eager:

```
warning: Rust code block is empty
  --> crates/parser/src/grammar.rs:16:5
   |
16 |   //! ```
   |  _____^
17 | | //! // test function_with_zero_parameters
18 | | //! // fn foo() {}
19 | | //! ```
   | |_______^
   |
help: mark blocks that do not contain Rust code as text
   |
16 | //! ```text
   |     ^^^^^^^
```

https://github.com/rust-lang/rust/pull/79816 should make this
configurable so the warning can be `allow`ed.