str: Make docs consistently punctuated

[SamWhited]

Every so slightly pointless one character PR, but this was driving me nuts while reading the docs a moment ago (all the other public structs have descriptions that end in a full-stop).

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @BurntSushi (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

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[frewsxcv]

Thanks!

@bors r+ rollup

[bors]

📌 Commit e3da6e7 has been approved by frewsxcv

[SamWhited]

Travis apparently hiccuped, but I don't have the authority to kick it.

[frewsxcv]

I could kick it, but I'm pretty sure your changes won't fail any tests, so probably not worth a two hour build ;)

Your changes are queued up to be tested and merged: https://buildbot2.rust-lang.org/homu/queue/rust

[bors]

🔒 Merge conflict

[bstrie]

I've rebased this branch due to a submodule conflict that invalidated many PRs.

[bstrie]

@bors r+ rollup

[bors]

📌 Commit 49db656 has been approved by bstrie