Auto merge of #8561 - icorbrey:git-dep-lock-docs, r=ehuss

Emphasize git dependency version locking behavior.

Added a section in `src/doc/src/reference/specifying-dependencies.md` clarifying Cargo's version locking behavior when adding `git` dependencies:

```
Once a `git` dependency has been added, Cargo will lock that dependency to the
latest commit at the time. New commits will not be pulled down automatically
once the lock is in place. However, they can be pulled down manually with
`cargo update`.
```

Resolves #8555