docs(core): Add flush-related docstrings and comments

[lobsterkatie]

In the process of trying to solve the problem of broken nextjs API routes on Vercel, I had to dig into the flush code, to figure out if it was causing the problems we were seeing. Spoiler alert: It wasn't. But since I'd already annotated the code for myself, I figured I might as well spare the next reader of that code some time if possible.

In addition to docstrings and comments, the only changes are:

• s/_isClientProcessing/_isClientDoneProcessing: This resolves to true when processing is done, and false if processing is still happening, which is the opposite of what the original name implied.
• s/_processing/_numProcessing: Since it represents a number, might as well make that obvious in the name, lest the casual reader think it might be a boolean or a collection of things currently being processed.
• s/ready/clientFinished in flush: This provides a nice parallel to transportFlushed. Also, the client may well be "ready," but since this is mostly called as part of a shutdown procedure, the fact that it's ready matters less than the fact that it's finished with the work it was doing.

[github-actions]

size-limit report

Path	Size
@sentry/browser - CDN Bundle (gzipped)	21.43 KB (+0.01% 🔺)
@sentry/browser - Webpack	22.46 KB (+0.03% 🔺)
@sentry/react - Webpack	22.49 KB (+0.03% 🔺)
@sentry/browser + @sentry/tracing - CDN Bundle (gzipped)	28.93 KB (+0.01% 🔺)