Add flush policy docs

[niallzato]

Proposed changes

Add flush policy docs, copied from here: https://github.com/segmentio/analytics-react-native#controlling-upload-with-flush-policies

[rchinn1]

Hey @niallzato - thanks for submitting this. I've left some comments (and a few questions) to align this a bit more with our style guide. After that, we'll add this to our docs!

[rchinn1]

Do the instances of we in lines 377-380 all refer to Segment?

[niallzato]

@rchinn-segment thanks for all the suggestions. Correct, we are referring to Segment for these instances. Specifically, the Segment library initialized in the app. We could say something like the following, perhaps:

// that way the Segment client won't keep attempting to send events to Segment, but they will still // be stored for future upload. // If the network comes back up, the Segment client adds the policies back

[rchinn1]

Sounds great @niallzato - will get these updates in our upcoming docs deploy!

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