Apart from the issue of needing permission to do so, as I contributor I would feel more confident about suggesting a revision through a pull request, as opposed to committing a revision. Someone more familiar with the documentation would be able to ensure that conventions are appropriately followed, that the revision doesn't change the semantics in a way that is incorrect or inappropriate, etc. Basically, all the same benefits of any code review, but applied to documentation.
Having used a couple of open source products with freely editable documentation wikis, I'd say that even a minimal process of reviewing and merging changes is more likely to result in a consistent and useful document than simply allowing community edits. Again, just my 2 cents.
carl