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.
Fixed incorrect module path
extra::arc -> sync::arc
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() {}
```
libcore: Document math intrinsics.
auto merge of #14362 : zecozephyr/rust/docfixes, r=cmr

extra::arc -> alloc::arc
auto merge of #14372 : neeee/rust/intrinsic-docs, r=brson