doc: nits and fixes for thread API

[tshepang]

No description provided.

[rust-highfive]

r? @alexcrichton

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[huonw]

Thread still only represents already-running threads.

[tshepang]

I removed Already-running because it doesn't work well with the line that follows it:

By spawning a new thread, e.g. using the thread::spawn function.

[alexcrichton]

I don't think this comma is necessary

[tshepang]

If the comma is removed, the sentence will have to be slightly re-organised by starting with "The application...". Otherwise, the pause feels very needed.

[jxcl]

@alexcrichton The comma usage here is correct. See https://owl.english.purdue.edu/owl/resource/607/02/ item 2.

[steveklabnik]

Some people leave it in, some leave it out. I generally put it in. It's fine either way.

[alexcrichton]

r? @steveklabnik

I may not be the best wordsmith

[steveklabnik]

@bors: r+ fa4cb49 rollup

[tshepang]

wow, that felt long; thanks @steveklabnik