Documentation warnings

[GinoGiotto]

When executing cargo doc --lib --no-deps these warnings appear:

warning: unresolved link to `StartMathMode`
  --> src\comment_parser.rs:38:19
   |
38 |     /// Between [`StartMathMode`] and [`EndMathMode`],
   |                   ^^^^^^^^^^^^^ no item named `StartMathMode` in scope
   |
   = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`
   = note: `#[warn(rustdoc::broken_intra_doc_links)]` on by default

warning: unresolved link to `EndMathMode`
  --> src\comment_parser.rs:38:41
   |
38 |     /// Between [`StartMathMode`] and [`EndMathMode`],
   |                                         ^^^^^^^^^^^ no item named `EndMathMode` in scope
   |
   = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`

warning: unresolved link to `MathToken`
  --> src\comment_parser.rs:39:53
   |
39 |     /// there will be no comment items other than [`MathToken`].
   |                                                     ^^^^^^^^^ no item named `MathToken` in scope
   |
   = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`

warning: unresolved link to `StartMathMode`
  --> src\comment_parser.rs:42:19
   |
42 |     /// Between [`StartMathMode`] and [`EndMathMode`],
   |                   ^^^^^^^^^^^^^ no item named `StartMathMode` in scope
   |
   = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`

warning: unresolved link to `EndMathMode`
  --> src\comment_parser.rs:42:41
   |
42 |     /// Between [`StartMathMode`] and [`EndMathMode`],
   |                                         ^^^^^^^^^^^ no item named `EndMathMode` in scope
   |
   = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`

warning: unresolved link to `MathToken`
  --> src\comment_parser.rs:43:53
   |
43 |     /// there will be no comment items other than [`MathToken`].
   |                                                     ^^^^^^^^^ no item named `MathToken` in scope
   |
   = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`

warning: unresolved link to `Database::usage_pass`
   --> src\database.rs:645:29
    |
645 |     /// Returns `None` if [`Database::usage_pass`] was not previously called.
    |                             ^^^^^^^^^^^^^^^^^^^^ the struct `Database` has no field or associated item named `usage_pass`

warning: unresolved link to `TypesettingData::html_bibliography`
   --> src\verify_markup.rs:524:7
    |
524 | /// [`TypesettingData::html_bibliography`],
    |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no item named `TypesettingData` in scope

warning: unresolved link to `TypesettingData::ext_html_label`
   --> src\verify_markup.rs:525:34
    |
525 | /// while references after the [`TypesettingData::ext_html_label`] go to the
    |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no item named `TypesettingData` in scope

warning: unresolved link to `TypesettingData::ext_html_bibliography`
   --> src\verify_markup.rs:526:7
    |
526 | /// [`TypesettingData::ext_html_bibliography`].
    |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no item named `TypesettingData` in scope

warning: `metamath-knife` (lib doc) generated 10 warnings
    Finished dev [optimized + debuginfo] target(s) in 3m 04s

It was proposed to add a CI check to ensure this won't happen again https://github.com/metamath/metamath-knife/issues/147#issuecomment-1823953546.