_alerts: Additional cleanup of new alert message

This PR is based on [additional feedback](https://github.com/bitcoin-dot-org/bitcoin.org/pull/2666#issuecomment-423424702)
and makes the following changes to the 0.16.3 upgrade notice:

+ Fixes a typo in the second to last paragraph of the alert
+ Makes some additional simplifications to the text
+ Drops link to reddit and links to GPG, release signatures and signing key
+ Consolidates download links to bitcoin.org/en/download
+ Adds emphasis (bold) to indicate stored funds are not at risk and links to value overflow incident

This is scheduled to be merged on Sunday, September 23rd (48 hours).

_alerts/2018-09-21-required-upgrade.md

@@ -13,34 +13,36 @@ date: 2018-09-21
 [0.16.3](https://bitcoin.org/en/download) was recently released, and if you're
 running a node and haven't already updated, we recommend you do so as soon as
 possible. A bug fixed in 0.16.3 [is more severe](https://bitcoincore.org/en/2018/09/20/notice/)
-than was previously made public. You can download 0.16.3 from [bitcoin.org](https://bitcoin.org/en/download)
-or [bitcoincore.org](https://bitcoincore.org/en/download/) or via
-[BitTorrent](https://www.reddit.com/r/Bitcoin/wiki/bitcoin_core_bittorrent),
-and as always, make sure that you
-[verify the download](https://www.reddit.com/r/Bitcoin/wiki/verifying_bitcoin_core).
+than was previously made public. You can download 0.16.3 from
+[bitcoin.org](https://bitcoin.org/en/download). As always, make sure that you
+verify the authenticity of the download with [GPG](https://gnupg.org/) using the
+[signing key](https://bitcoin.org/laanwj-releases.asc) and
+[release signatures](https://bitcoin.org/bin/bitcoin-core-0.16.3/SHA256SUMS.asc).
 
-Stored funds are not at risk, and never were at risk. Even if the bug had been
-exploited to its full extent, the theoretical damage to stored funds would have
-been rolled back, exactly as it was in the value overflow incident. However,
-there is currently a small risk of a chainsplit. In a chainsplit, transactions
+**Stored funds are not at risk**, and never were at risk. Even if the bug had
+been exploited to its full extent, the theoretical damage to stored funds would
+have been rolled back, exactly as it was in the
+[value overflow incident](https://en.bitcoin.it/wiki/Value_overflow_incident).
+However, there is currently a small risk of a chain split. In a chain split, transactions
 could be reversed long after they are fully confirmed. Therefore, for the next
 week or so you should consider there to be a small possibility of any
 transaction with less than 200 confirmations being reversed.
 
-Summary of action items:
+Below is a summary of required action items:
 
 + You should not run any version of Bitcoin Core other than 0.16.3. Older versions
 should not exist on the network. If you know anyone who is running an older
-version, tell them to upgrade it ASAP.
+version, tell them to upgrade it as soon as possible.
 
 + It's not necessary to immediately upgrade older versions if they are
 currently shut down. Cold-storage wallets are safe.
 
-+ For the approximately the next week, consider transactions with fewer than 200
++ For approximately the next week, consider transactions with fewer than 200
 confirmations to have a low probability of being reversed (whereas usually
 there would be essentially zero probability of transactions having six or more
 confirmations being reversed).
 
-Please watch for further news. If a chainsplit happens, action may be required.
++ Please watch for further news. If a chain split happens, additional action
+may be required.
 
-Read more: [CVE-2018-17144 Full Disclosure](https://bitcoincore.org/en/2018/09/20/notice/)
+More information: [CVE-2018-17144 Full Disclosure](https://bitcoincore.org/en/2018/09/20/notice/)