[docs] Update SecureStore docs for SDK 21 (currently unversioned)
[docs] Update SecureStore docs for SDK 21 (currently unversioned)

Updated the docs to reflect the new API:

- set/get/deleteItemAsync
- getItemAsync doesn't throw if the key is missing

Updated some formatting as well:
- use nested lists to describe iOS and Android options/behavior
- en dash for hyphenation

exponent/exponent,exponent/exponent,exponent/exponent,exponentjs/exponent,exponentjs/exponent,exponentjs/exponent,exponentjs/exponent,exponent/exponent,exponent/exponent,exponent/exponent,exponentjs/exponent,exponentjs/exponent,exponent/exponent,exponentjs/exponent,exponent/exponent,exponentjs/exponent,exponent/exponent,exponentjs/exponent