docs(quinn): Revise SendStream::write docs

[gretchenfrage]

Aligns with 100 columns, attempts to improve clarity, takes inspiration from tokio::io::AsyncWriteExt docs.

[Ralith]

Initial bikeshedding:

• Why introduce "resolves to" in addition to "yields"?
• Why replace "cancel-safe" with "cancellation safe"?

[flub]

* Why replace "cancel-safe" with "cancellation safe"?

This mirrors how tokio talks about it. They use a section header named Cancel safety and in prose use "This method is cancellation safe".

[djc]

@gretchenfrage want to polish this up so we can merge it?

[gretchenfrage]

@djc Yeah, I'd like to prioritize #2168 personally because I don't like how long I've been blocking it. I haven't lost track of anything, I just haven't had capacity for a while.

[djc]

@djc Yeah, I'd like to prioritize #2168 personally because I don't like how long I've been blocking it. I haven't lost track of anything, I just haven't had capacity for a while.

Would be nice to get this over the finish line.

[gretchenfrage]

My revision of this PR is in-progress, I'm hoping I can put it back into review tomorrow. I appreciate your patience with me as my capacity has been reduced a lot recently.

[gretchenfrage]

• Rebased over main
• Changed the first lines of comments to be imperative, for stylistic consistency with rest of project
• No longer starting first line of docs with "Attempt to" (Tokio sometimes does that but sometimes not. My reasoning for removing it is that we don't need to explicitly say that a method which returns a Result might fail any more than we need to explicitly say that a method which is async returns a future, at least within the first line of the doc comment.)
• Take flub's suggestion to use Tokio-inspired # Cancel safety headers
• Slight tweaks / corrections to my own grammar
• Take Ralith's suggestion to replace phrase "resolves to" with "yields"
• No longer rearranging order of methods