fix(keyword_docs.rs): use of as keyword to disambiguate paths

[iampi31415]

• The file keyword_docs.rs was missing documenting the use of the as keyword to disambiguate paths in trait implementations.

This is also described in the issue linked below.

There isn't a tracking issue, but there is an issue here.

Some details:

• I could include a link to the RFC section https://github.com/rust-lang/rfcs/blob/master/text/0132-ufcs.md#paths-that-begin-with-an-assoc_segment, if you think that it's appropriate.
• The style was inspired by the for docs

[rustbot]

r? @ChrisDenton

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[fmease]

Coincidentally someone already opened a PR a week ago: #142670.

Not sure if we should close this PR in favor of the other one or if we should wait for the older one to get merged first and then recontextualize this PR to act as a follup up / fixup.

Anyway @rustbot blocked

[iampi31415]

@fmease I hadn't noticed, that's bad luck. From what I can tell this fix seems more complete and quite different content. I don't know what's the best path forward.

[iampi31415]

Current built page looks like this:

[tshepang]

can you squash these commits, given that most of them are related fixups

[iampi31415]

can you squash these commits, given that most of them are related fixups

@tshepang yes. Is that okay? (should be a single commit now.)

[RalfJung]

Please give this PR a description that summarizes what happens here. "I fixed something in that file" is not super helpful :) Please mention at least the 'as' keyword.

[iampi31415]

@RalfJung thanks, sorry about that. Is now fine or still not ideal?

I changed both description and title.