fix(core): Stop rejecting in flush and close when client undefined

[lobsterkatie]

Currently, if the SDK has no client defined, calling Sentry.close() or Sentry.flush() will result in a promise rejection. Since this is not an error per se, this PR changes that rejection to a logged warning and a promise resolution with false. It also fleshes out the docstrings on all of the downstream methods (all of which essentially do the same thing - wait for the event queue to be empty, or timeout if it takes too long).

Fixes #3031
Fixes #3746

[lobsterkatie]

@kamilogorek - I realized after the fact that in the linked issue you'd suggested logger.error rather than logger.warn. Do you feel strongly about the level? I can go back and change it if so.

[github-actions]

size-limit report

Path	Size
@sentry/browser - CDN Bundle (gzipped)	21.53 KB (+0.13% 🔺)
@sentry/browser - Webpack	22.53 KB (0%)
@sentry/react - Webpack	22.56 KB (0%)
@sentry/browser + @sentry/tracing - CDN Bundle (gzipped)	28.95 KB (+0.12% 🔺)

[kamilogorek]

I'm fine with warn. Especially that we use it in logAndExitProcess in flush chain.