docs: assorted minor e.g. formatting improvements

[sadielbartholomew]

I've recently started development work using Tornado for the first time & naturally have been examining the documentation in detail to understand the internals.

I'm pedantic so along the way picked up on some (very) small issues e.g. in formatting (mainly missing capitalisation & plain text instances which would be more appropriate & readable as inline literals) which I noted down within an open branch.

I thought I might as well open this PR should you be happy to accept various such tweaks, or indeed a subset of them as I am happy to remove any changes that are unwanted. However if this seems excessive I am happy to close the PR.

[ploxiln]

I'm not sure how the whole thing will be received, but there's at least few good fixes in there. To get tests passing I think you'll just have to deal with:

.../tornado/docs/guide/coroutines.rst:66: WARNING: Unexpected indentation.
.../tornado/docs/guide/coroutines.rst:67: WARNING: Block quote ends without a blank line; unexpected unindent.
.../tornado/docs/guide/coroutines.rst:74: WARNING: Unexpected indentation.
.../tornado/docs/guide/coroutines.rst:77: WARNING: Block quote ends without a blank line; unexpected unindent.

[sadielbartholomew]

@ploxiln sure, thank you for your rapid response.

I've put suggestions in as a batch for convenience but can revert any as requested. I develop with Python 3.6 so didn't see any Sphinx-based failures locally but will address these Travis failures & squash in a commit to fix them.

[bdarnell]

Thanks, this looks great!