Improve docs of BatchController

[vorner]

• Clarify safety of the create method.
• Link directly to the example on github; this saves some time searching for it and it also avoids confusion in which repository the example lives, as the trait is re-exported in specs.

Related to https://github.com/amethyst/specs/issues/689.

Please do check that what I state here is correct. It's gathered from reading other documentation and trying to understand it as a user of the library, but I didn't write the code.

I'm not updating the changelog, since there's no actual code change, just some documentation improvements. If you think this still requires an entry, I'll add it.

Also, am I right to assume that if I register a batch that contains some thread-local systems, they'll still be handled as !Send internally and nothing will allow the user to move them to a different thread, etc?

[azriel91]

bors r+

[bors[bot]]

Build succeeded:

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