Spelling/doc formatting fixes.
syntax: put the correct span on doc-comments inside a module.

The position of the .bump call (before extracting the span fields) was
causing a doc-comment to have the span of the next token, not itself.
auto merge of #14357 : huonw/rust/spelling, r=pnkfelix

The span on a inner doc-comment would point to the next token, e.g. the span for the `a` line points to the `b` line, and the span of `b` points to the `fn`.

```rust
//! a
//! b

fn bar() {}
```