Improve 'Higher-ranked trait bounds' section

[Havvy]

(Originally filed by @jeekobu as rust-lang/rust#54202)

https://doc.rust-lang.org/reference/trait-bounds.html

The last paragraph in particular has English errors and is incomprehensible:

Higher-ranked lifetimes may also be specified just before the trait, the only end of the following trait instead of the whole bound. This function is difference is the scope of the lifetime parameter, which extends only to the equivalent to the last one.

[matthewjasper]

It looks like two lines have been swapped. I'll create a PR to fix this.

[Havvy]

@jeekobu Does #417 fix this for you, or do you have anymore specific criticism about the section?

[jeekobu]

I'm still not entirely clear where HRTBs apply or do not apply. But I suspect that's more a problem with my own understanding rather than this documentation. (I was trying to get a HRTB to apply to an argument that was itself a reference, but in the meanwhile found that I didn't truly need them for the problem at hand.)

So that being said, #417 does fix the problem I had. One minor suggestion on the examples, though: Because "[t]his function is equivalent to the last one" (as the example only has one trait being bound), it took a little bit of staring and thinking to understand the difference between where for<'a> F: and where F: for<'a>. (But it does make sense now.)